Interesting bug.

I lvled up a hero and now he is infinitely cloned.

Also have another hero that I cannot select. He has effectively disappeared!

Joy!

I have played this game a lot myself, and I've only encountered this once. It seems, that your save is corrupted, or (if you played on LAN) something went wrong while transferring data. I don't think this is connected to cheats. Just make a new game, and that's it.