Deduplicate label token results

Chad Lavimoniererequested to merge
9453-identical-subgroup-labels-are-duplicated-at-group-level-in-the-issue-tracker-filter-dropdown into master

What does this MR do and why?

In group contexts, multiple identical labels might be present in the labels search dropdown (each in a separate project). Clicking on any of them will filter on all of them (i.e., results that match any of the various project labels that have the same name), so we might as well deduplicate the list and only show one of them.

References

Screenshots or screen recordings

Before After

Screenshot_2025-06-27_at_13.50.11

Screenshot 2025-06-27 at 13.51.22.png

Screenshot_2025-06-27_at_13.49.48

Screenshot_2025-06-27_at_13.49.06

Screenshot 2025-06-27 at 13.52.38.png

(no change)

Screenshot 2025-06-27 at 13.51.56.png

How to set up and validate locally

  1. Check out this branch
  2. Inside a group, make sure two different projects each have the same label (but give them different colors to make them easy to tell apart)
  3. Make sure there is at least 1 issue with each of the two labels applied
  4. Visit the group's issue page
  5. In the filtering bar, start filtering on labels
  6. You will only see one of the two duplicate labels in the list (this is the main change in this MR)
  7. filtering on that label will show all the issues that have any label with that name applied (i.e., all the results for both duplicate project labels). Note that this is the current prod filtering behavior, this has not changed.

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Related to #9453 (closed)

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