Date: 202104
Local time of day: 0001-0600
Location: ZZZ.TRACON, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Second aircraft: Commercial Fixed Wing
Reporter organization: Government
Reporter function: Approach
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Workload; Situational Awareness; Confusion; Communication Breakdown
Anomaly: ATC Issue All Types; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Returned To Clearance
Contributing factors: Staffing; Environment - Non Weather Related; ATC Equipment / Nav Facility / Buildings; Human Factors
Primary problem: Human Factors

Narrative: I was working a fairly busy session at the beginning of my mid shift. Aircraft X checked on to the NE of ZZZ and was given a heading of 280 and descended to 80; which they read back correctly. Aircraft Y was being vectored from the SW of ZZZ and was descended to 60; which they read back correctly. A little while after descending Aircraft Y I noticed that Aircraft X was at 60 just about to exit the NW corner of the 65 MVA entering the 60 MVA. When I realized that Aircraft X was at the unexpected altitude; there was not enough time to issue a safety alert before they entered the 60 MVA (which was fine for their current altitude).I talked to the pilot of Aircraft X and they indicated that they were sure that they had read back to descend to 60. I reviewed the incident multiple times and only heard Aircraft Y confirm that they were descending to 60. However; both aircraft were on separate frequencies and there could have been a 'stepped on issue' that caused Aircraft X's incorrect read back to not get through to me.There are a couple of things I will work on in the future:- The call signs of Aircraft X and Aircraft Y were similar enough that I should have informed them; that they were both on separate frequencies. I will work on making that a priority in the future.- Aircraft X descended 2;000 feet before I realized the deviation. I was fairly tired after a long work week and think under normal circumstances my scan would have caught the deviation. I should have recognized I was not operating at 100% and asked for an extra set of eyes from the ZZZ [TRACON] Controller. In the future I will work on recognizing this and ask for help.- I don't normally put in assigned altitudes on IFR aircraft data blocks but after this incident I will probably start. Keeping up with strip marking can be difficult when busy and take your eyes away from the scope. Maybe having the altitude in the data block may have helped me recognize Aircraft X was descending past their assigned altitude.- I don't normally come in for XA:15 mid shifts (Usually start at XC:15 or XB:15) and should have asked to work a slower position knowing I was fatigued and on my 6th day.From a technology standpoint there are a couple of issues:- I didn't ever see the Low Altitude (LA) alert go off when I reviewed the falcon replay. The LA indication would have definitely got my attention to issue a safety alert. In my experience working and training it seems like it rarely goes off when it is actually needed. That alert system needs to be revamped; it could be a very useful and a life saving tool!- I wish we could somehow keep everyone on one frequency when we are combined. I think there may have been some kind of 'stepped on' issue with this situation; that one frequency may have helped with.Our staffing has been really bad lately and controllers including myself have been working lots of overtime since last fall. I know management doesn't have many options to alleviate the problem and I don't mind helping out with working overtime. However; here are some recommendations that might help:- We have X supervisors that are certified and can work positions. With COVID vaccines going on they should absolutely be used as staffing until we get people through vaccines and some stability in our staffing. I feel like having well rested controllers that are working less overtime is way more beneficial to safety; then having a stand-alone supervisor in the operation as often possible. Once we get out of our current staffing slump; having stand-alone supervision is a great idea! However; it is currently NOT A GOOD use of our resources.- Plan ahead and be proactive. The entire facility was aware over a month ago that controllers were going to be eligible for COVID vaccines in March. However; there was nothing done to try and stay ahead of all the sick hits that we're going to be happening. There could have been COVID vaccine signup sheets back in January or February to find out who wanted it right away; who would wait; and who didn't want it at all; then plan the schedule based off of that. In addition; the union put a schedule together well ahead of the vaccine eligibility to help mitigate the problem; but it was not implemented for some reason. As a result of no planning or forward thinking; for the past few weeks staffing has been very unstable. There have been many last minute changes to controllers schedules with extra OT to make things work. This is not a safe way to run things and could have been avoided.




Synopsis: Controller reported similar calls signs may have contributed to an aircraft taking another aircraft's assigned altitude. Controller also cited staffing issues as a contributing factor.