Fail explicitly in storage when asking for duplicate keys

[MaxiBoether]

When debugging #529 I realized this is a silent problem (we might access memory of a vector that is not allocated), so we should just explicitly fail.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.43%. Comparing base (c52a3d6) to head (e801778).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #530   +/-   ##
=======================================
  Coverage   82.43%   82.43%           
=======================================
  Files         214      214           
  Lines       10039    10039           
=======================================
  Hits         8276     8276           
  Misses       1763     1763           

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[github-actions]

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