Steps to reproduce

1. Log in to back office.
2. Go to Content / Content types / Content.
3. Edit content type e.g Folder
4. Add two ibexa_taxonomy_entry_assignment field types.
5. Set 'Taxonomy' for each added field types to 'Tags'.
6. Publish.

Actual result:

User can add two ibexa_taxonomy_entry_assignment field types with taxonomy 'Tags'.
Missing validation.

Expected result:

User cannot add two ibexa_taxonomy_entry_assignment field types with taxonomy 'Tags'.
Implemented validation.