Occasionally Minecarts with Chest generate duplicated as part of mineshafts.
Exact reproduction steps are unknown.
This seems to be restricted to one region file (see comment).
Abandoned mineshaft generates multiple loot minecarts in one place
Minecarts with Chest in Abandond mineshafts spill out duplicates of items in chest.
2 Minecarts with chests will spawn on the same rail with identical contents
World Duplication Glitch