fix index error on double addition of the same related model

[collerek]

Resolves: #1335

[codspeed-hq]

CodSpeed Performance Report

Merging #1346 will not alter performance

_{Comparing fix-double-adding-of-the-same-related (1871aca) with master (ef705cc)}

Summary

✅ 84 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (fbde811) to head (1871aca).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1346   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          204       205    +1     
  Lines        14845     14904   +59     
=========================================
+ Hits         14845     14904   +59     

Files	Coverage Δ
ormar/relations/relation_proxy.py	100.00% <100.00%> (ø)
tests/test_hashes/test_many_to_many.py	100.00% <100.00%> (ø)