Fix missing cancellations and duplicate diagnostics

[perrinjerome]

No description provided.

[codecov-commenter]

Codecov Report

Merging #17 (0f61364) into main (0e28dac) will decrease coverage by 0.23%.
The diff coverage is 88.88%.

❗ Current head 0f61364 differs from pull request most recent head 8826409. Consider uploading reports for the commit 8826409 to get more accurate results

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
- Coverage   95.64%   95.40%   -0.24%     
==========================================
  Files          27       27              
  Lines        2296     2330      +34     
==========================================
+ Hits         2196     2223      +27     
- Misses        100      107       +7     

Impacted Files	Coverage Δ
server/buildoutls/buildout.py	95.53% <88.70%> (-0.94%)	⬇️
server/buildoutls/tests/conftest.py	100.00% <100.00%> (ø)

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