Check for duplicate flows

[tsmbland]

Description

Adds a check to make sure no process has multiple flows for the same commodity

Fixes #248

Type of change

• Bug fix (non-breaking change to fix an issue)
• New feature (non-breaking change to add functionality)
• Refactoring (non-breaking, non-functional change to improve maintainability)
• Optimization (non-breaking change to speed up the code)
• Breaking change (whatever its nature)
• Documentation (improve or add documentation)

Key checklist

• All tests pass: $ cargo test
• The documentation builds and looks OK: $ cargo doc

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.82%. Comparing base (4361d9f) to head (c6364b6).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #354      +/-   ##
==========================================
+ Coverage   95.74%   95.82%   +0.07%     
==========================================
  Files          27       27              
  Lines        3129     3182      +53     
  Branches     3129     3182      +53     
==========================================
+ Hits         2996     3049      +53     
  Misses         61       61              
  Partials       72       72              

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[alexdewar]

LGTM!