front: fix map markers react warning for duplicate keys

[SharglutDev]

fix #10810

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.85%. Comparing base (d1d32ae) to head (0c418fa).
Report is 26 commits behind head on dev.

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev   #10811      +/-   ##
==========================================
- Coverage   81.93%   81.85%   -0.08%     
==========================================
  Files        1083     1083              
  Lines      107559   107175     -384     
  Branches      741      741              
==========================================
- Hits        88131    87731     -400     
- Misses      19387    19403      +16     
  Partials       41       41              

Flag	Coverage Δ
editoast	74.38% <ø> (+0.05%)	⬆️
front	89.37% <100.00%> (-0.10%)	⬇️
gateway	2.18% <ø> (ø)
osrdyne	3.28% <ø> (ø)
railjson_generator	87.58% <ø> (+0.07%)	⬆️
tests	87.91% <ø> (ø)

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[Synar]

LGTM ^^

(I don't have an opinion on the index vs index+coord key, either choice seems fine to me)

[emersion]

Thank you!