Entity tree: prevent creation of repeated entity items

[chapulina]

Signed-off-by: Louise Poubel [email protected]

🦟 Bug fix

Summary

Broken off #793.

We should never get into this situation, and if we do, it means there's some internal problem. I'm not sure how to reproduce it, but it's a good idea to guard against it anyway.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #974 (7fb309a) into ign-gazebo3 (4a5a33d) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

❗ Current head 7fb309a differs from pull request most recent head 1abfd20. Consider uploading reports for the commit 1abfd20 to get more accurate results

@@               Coverage Diff               @@
##           ign-gazebo3     #974      +/-   ##
===============================================
- Coverage        77.86%   77.84%   -0.03%     
===============================================
  Files              221      221              
  Lines            12683    12687       +4     
===============================================
  Hits              9876     9876              
- Misses            2807     2811       +4     

Impacted Files	Coverage Δ
src/gui/plugins/entity_tree/EntityTree.cc	12.83% <0.00%> (-0.29%)	⬇️

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