What is a 2001 penny that has tails on both sides but has been stamped on one side with a heads also?

You might have a flip-over double-strike which would bring several dollars from an error collector, but you should be able to see at least a trace of heads and tails on both sides. If the tails image that is on the heads side is reversed, it is possible that someone laid another coin on top of yours and hit it with a hammer. If that's what happened you just have a damaged coin. Someone familiar with minting errors would have to examine it to be sure.