Raise ValueError in attach_subcommand() when the subcommand already exists.

[kmvanbrunt]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.51%. Comparing base (47c492b) to head (abdd76c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1645   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files          21       21           
  Lines        4754     4758    +4     
=======================================
+ Hits         4731     4735    +4     
  Misses         23       23           

Flag	Coverage Δ
unittests	99.51% <100.00%> (+<0.01%)	⬆️

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