Remove duplicated tokens in issuable search bar

What does this MR do?

Removes duplicated label tokens in issue search bar. Duplicated tokens are removed before form submit when we tokenize search input value.

Are there points in the code the reviewer needs to double check?

Check if it's expected behavior. Notice this MR only removes duplicated labels for now.

Why was this MR needed?

To deliver #27293 (closed)

Screenshots (if relevant)

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27293 (closed)