Custom category with same name transfers entities

### Expected behavior

If a category with the same name exists on an entity and that same category is created on a new entity, it should create a brand new blank category rather than just moving the old category to the new entity.

### Actual behavior

Okay, this is weird. I have a `stats` category that I added to creatures a long while ago. I went to go add a `stats` category to characters and it removed it from creatures but it was added to characters with all the existing attributes from creatures.

### Steps to reproduce the behavior

Create a `Stats` category with some custom attributes. Go to add a `Stats` category to a new entity. The new category should exist with the existing attributes but the old category will be gone.

Reproduce the issue by creating a Stats category with custom attributes on one entity, then creating a same-named category on another entity. Trace the category creation and entity-assignment flow, and verify that the new category is blank while the original category remains attached with its attributes.

Written by the indexing model from the issue text.