fix: Allow duplicate sub-assemblies

[nathaniel-brough]

Attempts to fix: #1962

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.82%. Comparing base (2e05819) to head (ad5cb92).

Files with missing lines	Patch %	Lines
cadquery/assembly.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1963      +/-   ##
==========================================
- Coverage   95.84%   95.82%   -0.03%     
==========================================
  Files          29       29              
  Lines        7922     7922              
  Branches     1191     1191              
==========================================
- Hits         7593     7591       -2     
- Misses        192      193       +1     
- Partials      137      138       +1     

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