Resolve "Fix labels dropdown with multiple same names"

What does this MR do?

• Disables merging of labels with the same names in the search bar and other dropdowns.

• In the group boards, this also allows adding of label lists with the same label name.

• Also fixes dragging issues into a label list with the same name as an existing label on the issue. Previously, the new label on the new list is not added.

What are the relevant issue numbers?

Closes #51994 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.