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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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