Fix duplicate entries in joint position controller GUI plugin

[iche033]

🦟 Bug fix

Fixes #2099

Summary

Prevents duplicate entries from being added to the Joint Position Controller GUI plugin.

Applies the patch from #2099. See #2099 for instructions and sdf file for reproducing the bug.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[codecov]

Codecov Report

Merging #2101 (d1afded) into gz-sim7 (f83456f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head d1afded differs from pull request most recent head 0394b87. Consider uploading reports for the commit 0394b87 to get more accurate results

@@           Coverage Diff            @@
##           gz-sim7    #2101   +/-   ##
========================================
  Coverage    64.99%   64.99%           
========================================
  Files          356      356           
  Lines        28933    28936    +3     
========================================
+ Hits         18805    18808    +3     
  Misses       10128    10128           

Files Changed	Coverage Δ
...int_position_controller/JointPositionController.cc	59.09% <100.00%> (+3.02%)	⬆️

... and 2 files with indirect coverage changes