fix: deduplicate fields to prevent Elixir 1.20 redundant clause warnings

[aptinio]

When a join or alias field shares a name with a schema field, fields/2 returns duplicate entries, causing redundant clause warnings in the generated field_info/2 and get_field/2 functions on Elixir 1.20.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.45%. Comparing base (bb96018) to head (e7f04ae).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #648      +/-   ##
==========================================
+ Coverage   84.44%   84.45%   +0.01%     
==========================================
  Files          15       15              
  Lines        1022     1023       +1     
==========================================
+ Hits          863      864       +1     
  Misses        159      159              

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[woylie]

Thank you!