In the

United States Court of Appeals

For the Seventh Circuit

Nos. 99-4016, 99-4135

Bethesda Lutheran Homes

 and Services, Inc., et al.,

Plaintiffs-Appellants/Cross-Appellees,

v.

Gerald Born, et al.,

Defendants-Appellees,

and

Thomas Schleitwiler, et al.,

Cross-Appellants.

Appeals from the United States District Court

for the Western District of Wisconsin.

No. 99-C-427-S--John C. Shabaz, Chief Judge.

Argued September 7, 2000--Decided January 25, 2001

 Before Bauer, Posner, and Evans, Circuit Judges.

 Posner, Circuit Judge. This dispute is before us

for the third, and we trust last, time. See

Bethesda Lutheran Homes & Services, Inc. v.

Leean, 122 F.3d 443 (7th Cir. 1997), 154 F.3d 716

(7th Cir. 1998). In the first round, Bethesda, a

private residential institution in Wisconsin for

the mentally retarded, along with several current

and would-be residents, brought suit under 42

U.S.C. sec. 1983 against a number of Wisconsin

state and local officials, plus Jefferson County,

where Bethesda’s facility is located, challenging

provisions of the federal Medicaid regulations,

and state law governing the establishment of

Wisconsin residency, as violations of the

constitutional right to travel. The Medicaid

provisions at issue (cited at 122 F.3d 447)

authorized states that participate in the

Medicaid program (half of the expense of which is

defrayed by the federal government, the other

half by the state) to decline to provide

assistance to otherwise eligible persons who are

not residents of the state, or even to residents

who have gone out of state for care, unless the

state of residency is unable to provide the

services they need. These provisions discouraged
certain nonresidents of Wisconsin from seeking

medical care in Wisconsin. That state would have

no obligation to enroll them in its Medicaid

program, while the state of their residency,

provided it was capable of treating them instate, would have no obligation to reimburse

their treatment costs in Wisconsin. They would be

stuck in their home state.

 All this would not have mattered had Wisconsin

allowed these nonresidents to become residents.

But the state law also challenged by the

plaintiffs prevented this. And although this

joint federal-state discouragement of medicalcare shopping might well have powerful fiscal or

other social merits to recommend it, we held that

the one-two punch violated the Constitution and

must be enjoined. We also held, however, that the

plaintiffs could not obtain monetary relief from

the defendant state officials insofar as they had

been sued in their official capacity, since such

suits are deemed to be suits against the state

itself, thus bringing the Eleventh Amendment into

play. We remanded for the entry of the injunction

and for consideration of the other relief sought

by the plaintiffs and not barred by the Eleventh

Amendment.

 The injunction was duly entered on remand.

Bethesda thus became entitled to Medicaid

reimbursement of future expenses incurred by it

in serving these individuals, but not to

reimbursement of expenses incurred before the

legal bars were removed by our decision; that

would have been monetary rather than injunctive

relief. The only monetary relief sought on remand

was against Jefferson County and was turned down

by the district court on the ground that a

municipal government cannot be held liable in

damages under section 1983 if it committed the

acts of which the plaintiff complains under the

compulsion of federal and state law. In their

second appeal the plaintiffs, while accepting the

principle on which they had been denied damages,

challenged its application to the facts, arguing

that the officials of Jefferson County had wanted

to deny these plaintiffs benefits regardless of

federal or state law. Affirming the judgment for

Jefferson County, we held that the county

officials’ state of mind was irrelevant in a case

such as this where the officials were given no

latitude by state or federal law to grant the

benefits sought; their state of mind played no

causal role in their actions.

 In struggling against the proposition that

Jefferson County was off the hook by virtue of

having acted under the compulsion of state law,

the plaintiffs had intimated that the county had

not been compelled after all. We had said the
opposite in our first opinion--specifically, that

state law had forbade the county to certify the

plaintiffs as county residents, a prerequisite to

their being entitled under Wisconsin law to

Medicaid benefits. In the second appeal the

plaintiffs simply ignored that ruling. (They

continue a pattern of selective forgetfulness in

this round as well, failing even to cite our

second opinion.) Yet that earlier ruling was the

law of the case, and if the plaintiffs wanted us

to depart from it they had to acknowledge the

ruling and give reasons for our abandoning it.

This they had failed to do, thus bringing into

play the principle that "unchallenged

determinations in a previous decision in the same

case unquestionably bind the court in a

subsequent appeal." 154 F.3d at 719.

 Rather than accept the second decision, Bethesda

brought a new case, the one before us today.

Damages are sought in this case against the

officials of the state and of Jefferson County

who refused Medicaid benefits to the eight

individuals who are coplaintiffs with Bethesda,

several but not all of whom were plaintiffs in

the previous suit, the one that was before us in

the two previous appeals. The contention in the

present suit is that the Medicaid regulations

challenged in the previous suit, rather than

unconstitutionally authorizing the defendants to

deny benefits to nonresidents, as the plaintiffs

had argued in that suit and we had agreed, had

commanded them to pay those benefits, and it was

in defying the command that the defendants had

violated the plaintiffs’ right to travel. The

district court dismissed the new suit primarily

on the ground that the defendants were entitled

to a qualified (that is, good-faith) immunity

from a suit for damages, as no rule of law that

was clearly established when they acted forbade

them to deny benefits to nonresident patients at

Bethesda’s facility. The judge found it

unnecessary to reach most of the other grounds

for dismissal urged by the defendants.

 The plaintiffs have appealed; and the defendants

cross-appeal from the district court’s refusal to

impose sanctions on the plaintiffs under Rule 11

of the Federal Rules of Civil Procedure. The

judge thought the plaintiffs entitled to bring

this suit for the purpose of urging us to

overrule our previous decisions holding that

state and federal law had unconstitutionally

prevented the award of benefits. Unless we do so,

the Eleventh Amendment, the principle that a

county is not suable under 42 U.S.C. sec. 1983 in

respect of acts done by it under compulsion of

state or federal law, and the principle of

qualified immunity combine to bar the plaintiffs

from obtaining any monetary relief.
 The appeal is frivolous on so many grounds that

it is difficult to know where to begin. If the

plaintiffs in this second suit were identical to

the ones in the first, or in privity with those

plaintiffs, then it would be obvious that the

present suit was barred by the principle of res

judicata, because the claim in the two suits is

the same--that the state and county officials had

denied the plaintiffs Medicaid benefits in

violation of law. Not that all the arguments are

the same, but for purposes of res judicata a

claim is not an argument or a ground but the

events claimed to give rise to a right to a legal

remedy, e.g., United States v. County of Cook,

167 F.3d 381, 383 (7th Cir. 1999); Brzostowski v.

Laidlaw Waste Systems, Inc., 49 F.3d 337, 339

(7th Cir. 1995); Prochotsky v. Baker & McKenzie,

966 F.2d 333, 335 (7th Cir. 1992); Wilkins v.

Jakeway, 183 F.3d 528, 535 (6th Cir. 1999); Woods

v. Dunlop Tire Corp., 972 F.2d 36, 39 (2d Cir.

1992), and they are the same in the two suits.

 The plaintiffs are not identical, but this turns

out to make no difference. First, insofar as

Bethesda is concerned and those individual

plaintiffs who were plaintiffs in the first suit

as well, they cannot avoid the bar of res

judicata by bringing in additional plaintiffs.

Dreyfus v. First National Bank, 424 F.2d 1171,

1175 (7th Cir. 1970); United States ex rel.

Robinson Rancheria Citizens Council v. Borneo,

Inc., 971 F.2d 244, 249 (9th Cir. 1992). The

defense of res judicata is not avoided by

joinder. Second, the newly added individuals (and

this is doubtless true of the overlapping

individual plaintiffs as well, though that is

unnecessary to decide) are not proper parties,

because they lack standing to sue, having no

stake in the litigation. As the plaintiffs’

lawyer acknowledged at argument, any recovery of

benefits in this suit will go to Bethesda, which

incurred the expenses of serving the individual

plaintiffs until the injunction that was issued

on remand from the first appeal gave them

prospective relief. The plaintiffs are indigent,

and Bethesda does not claim to be entitled to

obtain any money from them, so it is not as if

they were seeking monetary relief in order to

satisfy a debt to Bethesda. They are seeking no

relief.

 So the suit is barred by res judicata, but it is

also barred by the doctrine of judicial estoppel:

a party that has won a suit on one ground may not

turn around and in another case obtain another

judgment on an inconsistent ground. E.g., Saecker

v. Thorie, No. 00-2257, 2000 WL 1810097, at *4

(7th Cir. Dec. 12, 2000); Moriarty v. Svec, 233

F.3d 955, 962 (7th Cir. 2000); Lydon v. Boston
Sand & Gravel Co., 175 F.3d 6, 12-13 (1st Cir.

1999). The plaintiffs argued in the first suit

that the relevant Medicaid regulations and

Wisconsin state law were unconstitutional. Having

won that suit to the extent of getting the legal

obstacles to Medicaid reimbursement removed, they

could not turn around and in the next suit seek

additional relief by arguing that the regulations

and state law were constitutional after all and

compelled the defendants to grant them benefits.

It does not matter that the plaintiffs did not

win everything they sought in their first suit.

They won a judgment, and cannot now seek another

judgment on an inconsistent ground. The doctrine

of judicial estoppel would not apply to a new

party, one that had not benefited from the

judgment in the previous suit, but we have seen

that the only new plaintiffs in this, the second

suit, are actually not parties to it because they

have no stake in its outcome.

 The plaintiffs point out that the defendants did

not argue judicial estoppel in the district

court. But the doctrine is for our protection as

well as that of litigants, and so we are not

bound to accept a waiver of it. In re Cassidy,

892 F.2d 637, 641 (7th Cir. 1990); Motley v. New

Jersey State Police, 196 F.3d 160, 163 (3d Cir.

1999); cf. Rissetto v. Plumbers & Steamfitters

Local 343, 94 F.3d 597, 601 (9th Cir. 1996); 18

Moore’s Federal Practice sec. 134.34[1] (3d ed.

2000); but cf. United States for Use of American

Bank v. C.I.T. Construction Inc., 944 F.2d 253,

258 (5th Cir. 1991) (waiver binding except in

"egregious" case). Its purpose is to protect the

judicial system from being whipsawed with

inconsistent arguments and to discourage the form

of fraud that consists of withholding your best

ground in the first of a series of suits because

it is helpful to your opponent in that suit

hoping to win that suit on a different ground and

then spring your inconsistent best ground in a

later suit in order to obtain additional relief.

 There is more. The premise of the present suit

is that our previous decisions were wrong. The

plaintiffs argue that because there are new

parties, the previous decisions are not binding.

That premise is wrong, as we have seen, but

forget that. The plaintiffs’ lawyer does not

understand the doctrine of stare decisis. It is

res judicata that bars the same party from

relitigating a case after final judgment, and the

doctrine of law of the case that counsels

adherence to earlier rulings in the same case.

E.g., Vidimos, Inc. v. Wysong Laser Co., 179 F.3d

1063 (7th Cir. 1999); United States v. Becerra,

155 F.3d 740, 753 n. 15 (5th Cir. 1998); United

States v. Unger, 700 F.2d 445, 450 (8th Cir.

1983). It is stare decisis that bars a different
party from obtaining the overruling of a

decision. The existence of different parties is

assumed by the doctrine, rather than being

something that takes a case outside its reach. Of

course, stare decisis is a less rigid doctrine

than res judicata. But it is not a noodle. For

the sake of law’s stability, a court will not

reexamine a recent decision (our previous

decisions are two and three years old,

respectively) unless given a compelling reason to

do so. E.g., Joy v. Penn-Harris-Madison School

Corp., 212 F.3d 1052, 1066 (7th Cir. 2000);

Snajder v. INS, 29 F.3d 1203, 1207 (7th Cir.

1994); In re Patterson, 825 F.2d 1140, 1147 (7th

Cir. 1987); Thomas E. Hoar, Inc. v. Sara Lee

Corp., 900 F.2d 522, 527 (2d Cir. 1990); Brewster

v. Commissioner, 607 F.2d 1369, 1373 (D.C. Cir.

1979) (per curiam). The reason might be a

legislative change, a change in applicable

regulations, a judicial decision dealing with a

related or analogous issue, a change in the

social or economic context of the issue, or some

other important new information. Patterson v.

McLean Credit Union, 491 U.S. 164, 173-74 (1989);

United States v. Aman, 31 F.3d 550, 554 (7th Cir.

1994); Stewart v. Dutra Construction Co., 230

F.3d 461, 467 (1st Cir. 2000); Critical Mass

Energy Project v. NRC, 975 F.2d 871, 876 (D.C.

Cir. 1992) (en banc). The plaintiffs point to

nothing of that kind. They make the same

arguments that were made by the same counsel when

rehearing of our first decision was sought.

 The ground on which the district court dismissed

the claim for damages against the defendants in

their personal capacities, qualified immunity,

was solid too. (The claims against them in their

official capacity are barred by the Eleventh

Amendment and, regarding those defendants who are

employees of Jefferson County, by our previous

affirmance of the dismissal of the claim for

damages against the county.) Before we upheld the

constitutional challenge in the first suit there

was no case authority that made it unreasonable

for the defendants to assume the

constitutionality of their actions. Even more

clearly, they had no obligation to foresee and

accept the argument now made by the plaintiffs,

which is that the regulations that the plaintiffs

challenged in the previous suit actually

compelled the defendants to grant the benefits

sought rather than operating as a roadblock to

those benefits. An argument that the plaintiffs’

lawyer didn’t think good enough to make in the

first suit is now said to be so obviously sound

that the defendants were clearly unreasonable to

have failed to govern their behavior by it! 

 And for completeness we add that even if we were

minded to consider the plaintiffs’ new
substantive arguments, we would reject them. The

principal argument is that 42 C.F.R. sec.

435.403(m), which provides that "where two or

more states cannot resolve which State is the

State of residence, the State where the

individual is physically located is the State of

residence," clearly required Wisconsin to

recognize the plaintiffs as citizens of

Wisconsin. The precise meaning of this regulation

is unclear, and we can find no cases interpreting

it, but it clearly did not require the defendants

to award benefits to these nonresidents on the

ground that the regulation made them residents.

For there is no indication of a dispute with

another state. It is true that Illinois provided

a document to some of the plaintiffs stating that

they were not residents of Illinois, but it did

so, as the accompanying correspondence made

clear, as an accommodation to these plaintiffs,

to help them to obtain Medicaid benefits in

Wisconsin. There is no evidence that Illinois and

Wisconsin actually disagree over, let alone that

they cannot resolve, the issue of which state the

plaintiffs are residents of. If Illinois’

"helpful" action triggered the regulation, it

would empower any state, without evidence,

argument, or formalities, to force any other

state to treat the first state’s residents at no

expense to the "exporting" state, provided only

the residents were willing to travel to the other

state for treatment.

 We come last to the issue of sanctions. The

appeal, as we have said, is frivolous; but so was

the suit. Not because a party should be

sanctioned for seeking a change in law, in this

case a change in our ruling in the first appeal,

but because, whatever the possible merit of the

suit, it should have been obvious to any lawyer

that relief was barred on multiple grounds,

including res judicata, the Eleventh Amendment,

judicial estoppel, and qualified immunity. So

clear is this that we think it was unreasonable

for the district court to deny relief under Rule

11. That denial is reversed and the matter

returned to the district court for the assessment

of a proper sanction. In addition we direct

Bethesda to show cause within 21 days why

sanctions for the filing of a frivolous appeal

should not be imposed under Rule 38 of the

Federal Rules of Appellate Procedure and section

1927 of the Judicial Code.

Affirmed in Part, Reversed in Part,

Remanded with Directions, and

Order to Show Cause Issued.