Noticed this after the issue with MC-88922 and tried teleporting to specific UUID's to figure out the problem.

I'm not sure if it only effects old worlds as I can't test it atm but I have seen other people with the same issue on the other issue mentioned above.

Steps to reproduce.

Load up world in 15w39a-c.
open log to find a MinecartChest with UUID duplicates
Teleport to said UUID
See items on the floor.

This is a pretty big issue as they duplicate consistently and cause a load of tick lag.

It may only duplicate on load/unload haven't been able to test that either.

Link to video showing the minecarts items on the floor - https://youtu.be/nelMlajbi-4?t=476 (If it doesn't start at 7:56 skip to that time.)