No, they can't, and you could if necessary go to court to stop this or get the surplus returned. Garnishment is via court order; they can only take out as much as the order permits. If they fail to garnish one week, the garnishment may end up running a week longer at the end, to "catch up," but they can't take out more then authorized by the garnishment order. That said, going to court to stop or reverse this will take you away from work (court is only open during the normal work week), costing you money (you'll miss work); will cost you more money, due to the filing fees you'll have to pay; and will create ill will with your employer. If there is any way you can get by, as a practical matter, you are likely better off letting them to the double garnishment.