Use an Assembled Minecart on any rail, then recover it and right-click like crazy. And then you duplicate the Assembled Minecart and all of the contents within it(including the things that are in chests). Unfortunately, I don't have any pictures here. So please consider fixing it or make some changes. My server's players found this bug and I have to ban the Assembled Minecart.

I can't reproduce it from the description you gave. A few more infos would be helpful:

1. "recover it" Rail or Minecart? If minecart, sneak wrench or just wrench?
2. "right-click like crazy" how long after picking up the minecart? same frame, same tick, a few seconds later? How fast? Holding which item (wrench, picked up cart, empty hand)?
3. i guess the duplication is actual duplication and not just visual?

I never came accross this issue in my own testing, and to fix something i first have to know how to get that bug to occur.

I think I should DEFINITELY work on listening if players are lying to me or not......
Also, this duplicating issue does not trigger very often......
For your information:

1. "recover it" means just wrenching the Minecart.
2. "right-click like crazy" that means when you wrench the Minecart off the rail, you right-click like crazy.
3. It IS actual duplication, not like just a visual glitch.

I have also been unable to reproduce this bug. Could you provide us with an actual example of the bug occurring on your server?