Hi There,

I've had this same problem happen when I've converted to SubD, but want to go back to Polygons and hit undo instead of converting back. There might have been the odd time that this has occured during an actual convert... I was so frustrated, going back to older files all the time before the mess up. I also tried selecting all the vertices on the whole model and doing a merge vert with a low number, but all these seemed to take to long.

Anyways, I made a hotkey Shift + O for the Outliner, and check in there for extra geometry, then go thru and see what is highlighted when I click on various items... a good time to rename your objects if you haven't already Then just delete the duplicate.

Hope this Helps,