A successful entitydata, with white text output, will duplicate the villager inventory – after the text is displayed. The white displayed text is the old value, before the duplication. To see the duplication, you have to use another entitydata command. It's best to use a failed one, with red text, as red text doesn't cause further duplication.
Summon a Villager:
/summon Villager
Give him 3 seeds:
/replaceitem entity @e[type=Villager,c=1] slot.villager.0 wheat_seeds 3
Check his inventory with a failed, red entitydata command. Notice Count:3b (near the end of the NBT) for the 3 seeds:
/entitydata @e[type=Villager,c=1] {}
Do it again, and notice Count:3b again. Nothing has changed yet.
/entitydata @e[type=Villager,c=1] {}
Now use entitydata to change his name. The white text will display Count:3b, but it's lying!
/entitydata @e[type=Villager,c=1] {CustomName:TestGuy}
Use a failed, red command to check his inventory. It shows Count:6b.
/entitydata @e[type=Villager,c=1] {}
Do it again. Still Count:6b.
/entitydata @e[type=Villager,c=1] {}
Use another successful entitydata. It will display Count:6b, but it's lying!
/entitydata @e[type=Villager,c=1] {Glowing:true}
Use a failed, red command to check his inventory. It shows Count:12b.
/entitydata @e[type=Villager,c=1] {}
Do it again. Still Count:12b.
/entitydata @e[type=Villager,c=1] {}
Linked issues
is duplicated by 10
MC-126263
Villager inventory data tag has inconsistent behavior
Resolved
MC-159946
/data works incorrectly with villager inventory
Resolved
MC-164547
NoAI Duplicates Villager Inventory
Resolved
MC-191011
Piglin Drops Duplicate Items when using /data merge
Resolved
MC-195628
[/data command] Remove items of piglin inventorys doesn't work
Resolved
Comments 13
Editing nbt tags during the life time of an entity is not supported; and issues regarding that won't be fixed.
I don't understand the technical phrasing. Is that just a way of saying that the entitydata command is only partially supported?
It's not a specific tag that doesn't work. Any use of entitydata on a Villager will duplicate the inventory.
They fixed an /entitydata bug in MC-95738 similar to this one. Any change, even to CustomName, had side-effects. In that case, it reset the Slime's hp. In this case, it duplicates the villager's inventory.
I can't tell if you have special knowledge of exactly when Mojang cares about /entitydata bugs and when they don't, or if you're over-interpreting Serge's comment from 2014. At least in MC-95738, they did care about this type of bug. But perhaps they'd feel villagers duplicating their food isn't as important as slimes resetting their hp.
I made a command that modifies a piglin's angry_at memory. As this command is repeated every tick, the piglin's inventory quickly fills up. I suppose I'll need to use the silly snowball method to make the mob angry at a specific mob without this bug happening. Hopefully this gets fixed, even though the report is marked invalid.
While fixing MC-253076
They actually went through and fixed the others!
Fixed in: 22w42a
There isn't really "went through". The bugs are true duplicates, and if was fixed in 2017 none of the others would have happened. I only reported this to help. I thought it was selfish to understand a bug and say nothing, so I said something.
Mojang has given mods bad policy that doesn't distinguish "Some tags won't work if you try to change them," from "Bugs don't matter at all if they involve data commands." These are two very different things. (The bug is not about a tag.) Both this and MC-95738 (and who-knows what else) were fixed when devs saw them, but the policy tells mods not to let devs see them. There must be a better way to do this. Or with luck there are no more bugs like this...
The bug exists in 1.8 and 1.11 as well. I can't find a way to edit this into Affected Versions.