If you are talking about medical bills that your insurance company had previous paid for and now you are receiving money for from the other party's insurance carrier, thus duplicate payments for the same medical bills, then it is true you cannot be paid twice for the same expenses.

If instead you are talking about the at-fault party's insurance stating that you need to reimburse your insurance company for medical bills for some other reason, then there is not enough information given for us to determine if this would be a normal procedure or not.

It sounds as if your insurance company has already been involved in this insurance claim so you should contact them about them settling with the "other guy's" insurance company or at least asking them to guide you about how best to resolve this issue.