I got out of a minecart just after logging on (see my other bug report for why) and i jumped out, and the cart started to move, so i hit it, then i realized, it broke, but one was still on the track... so i broke that too, and i realized it was a dupe bug.

Steps: Ride in a minecart for a bit, (may require going into a chunk error unknown) then log out, back in, and hop out, and break the 2 carts.