[Bug]: Incomplete handling for tags with the same name

[kesselb]

Bug description

We received a report that some files marked as favorites are not displayed in the "Favorites" view. Additionally, users cannot remove the favorite state from these files. This issue is similar to #49395.

• The lib/private/Tags.php implementation does not properly verify if a given tag already exists. Although a check exists, the list of tags is loaded only once and never updated.
• The database table lacks a unique constraint, allowing multiple identical tags for the same user.

I've started working on patch: #49674

But adding that index on existing systems and ironing out the various issues I ran into while diving through the code needs more work, and therefore we would appreciate the files team taking over.

Steps to reproduce

1. Delete the favorite tag from oc_vcategory
2. Select 2 or more files
3. Mark the files as favorite (as batch action)

The code in lib/private/Tags.php does not check if the given tag already exists properly. There's a check, but the list of tags is loaded only once and never updated. Also, there's no unique constraint on the table, and therefore it's a valid state to have the tag more than once for a user.

Expected behavior

Tags with the same name should be handled gracefully.

[skjnldsv]

There's also this part that I did here: #53875
Relevant, but not exactly addressing the full scope of this issue

EDT: no, the favorites are not systemtags! I forgot we have different usage