This issue seems to occur when some serverside process lags. I have experienced this occurring on the RaccPack modded server, where I pick up a minecart contraption with a chest attached and it occasionally gives me a stack of two minecart contraptions.

The only other variable I can see is that it has a quark chest on it, and maybe somehow that modded inventory is what is causing the dupe bug. It doesn't happen every time though, so I'm not entirely certain that the chest is to blame.
Edit: Just tested it with a mob spawner contraption, and it can duplicate those too.

This issue has been marked as stale because it has been inactive for 3 weeks. It will be closed if it remains inactive for another 3 weeks.

This issue has been closed since it has been inactive for 3 weeks since it was marked as stale.

Does it still happen on a singleplayer world?