When using the setup in the attached picture if you put a Minecart in the right dispenser it will infinality duplicate but not in the left one. I think it only happens if the Minecart is put in via a hopper. I found this out the hard way when it flooded my automatic chicken farm with Minecarts.
Edit: Also it happened when I broke Minecrat with a chest on the track.
Duplicate of MC-5998 , please use the search function to see if your bug has already been submitted. Currently over 40% of tickets are being closed as duplicate.