The systems I've known use a sequence numbering scheme so if a "new" key is issued it invalidates any previous sequence when used. If I had to guess, I'd wonder if maybe the guests entered the rooms when housekeeping had the door open and they did not use their key. But more likely FDC did not program a new key but copied the previous key. If a copy was made, it's a serious security issue and should be addressed, although probably pretty unlikely, but possible, a previous guest would be returning to a room.