The bug
In previous versions, copying an entity with F3 + I removed the UUID from the NBT. With the migration from UUIDMost and UUIDLeast to UUID, this no longer happens, causing the generated command to fail if the original entity still exists.
How to reproduce
1. Spawn a mob
2. Look at it
3. Press F3 + I
4. Open a command block
5. Paste the command inside and activate it
→ ❌ Unable to summon entity (duplicate UUID)
Linked issues
relates to 3
Comments 2
ZYX_2D: That's the point. He's saying previously F3+I had the common sense to strip the UUID info out. Now it doesn't (although it's marked as fixed so I guess it does in 20w13a).
If you need to check the entity's UUID info you can use a Data Get command. In fact you don't need to press enter on the command, so long as the entity is close enough to swing a sword at you can use tab to auto-complete to it's UUID when telling the Data Get command which entity you want to get the data from.
tryashtar
Panda4994
I can't clearly catch what U mean. I think it's reasonable that in a single world, there's impossible that 2 or 3 entities have the same UUID. & also, I can use F3 + I 2 copy an entity's UUID info.