Fleet UI: Surface duplicate label name error to users

[RachelElysia]

Issue

Cerra #21875

Description

• Surface duplicate label name error to users on dynamic and manual label pages

Screenshot of fix

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
• Manual QA for all new/changed functionality

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.22%. Comparing base (8428f19) to head (bb674cf).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #22389   +/-   ##
=======================================
  Coverage   65.22%   65.22%           
=======================================
  Files        1496     1496           
  Lines      117247   117247           
  Branches     3566     3573    +7     
=======================================
  Hits        76474    76474           
  Misses      33636    33636           
  Partials     7137     7137           

Flag	Coverage Δ
frontend	52.35% <ø> (ø)

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