Fix Group labels load failure when there are duplicate labels present

What does this MR do?

Fixes a regression where labels loading was failing due to duplicate labels present in Group issues & boards.

Why was this MR needed?

This bug was caused due to invalid access to gl.DropdownUtils.duplicateLabelColor() where there was already an independent import for DropdownUtils was present.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Review
  • [ ] Has been reviewed by UX
  • Has been reviewed by Frontend
  • [ ] Has been reviewed by Backend
  • [ ] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [ ] Internationalization required/considered
• [ ] End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43598 (closed)