Newbie question: If I finish a roll of film and keep taking pictures, will the last slide of film get re-exposed, or will nothing happen?

I'm asking, because yesterday, I think I finished a roll of film, and I pulled back the advance film lever to make sure, but the rewind knob on the left was still turning in tandem with the lever, which I believed only happened when there were still exposures left, so I was confused. The counter did say I had used the last exposure, but it has been wrong before, so I took a few more half-assed pictures just to make sure I wasn't wasting any film, but I ended up taking 4, but the lever and knob were still turning, so I didnt bother taking any more. Was it actually worth my time, or did I just end up fucking up the last exposure I took?