Warn when deserializing a mapping that already exists

[UkoeHB]

It is best not to panic for non-critical errors.

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (7fc85f2) 95.64% compared to head (838e575) 95.31%.
Report is 1 commits behind head on master.

❗ Current head 838e575 differs from pull request most recent head 6a0a6b4. Consider uploading reports for the commit 6a0a6b4 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage   95.64%   95.31%   -0.33%     
==========================================
  Files          13       13              
  Lines         873      876       +3     
==========================================
  Hits          835      835              
- Misses         38       41       +3     

Files	Coverage Δ
src/client.rs	87.07% <0.00%> (-1.82%)	⬇️

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