BUG: Tags can be assigned more than one time.

The current version allows a tag to be assigne more than once which creates great confusing when filtering by tag through a join query.

No file, test, or entry point is named. Start by locating the tag-assignment and join-query filtering paths in the admin client, then reproduce assigning the same tag twice. Done means a tag cannot be assigned more than once and filtering through the join query no longer produces confusing duplicate results.

Written by the indexing model from the issue text.