Add namespace information to duplicate warnings

[janpaepke]

Why am I submitting this PR

The warning message for duplicate keys is of limited usefulness, if it concerns a key that exists in multiple namespaces.
For example: the title key exists in 10 namespaces. When I get a warning that it's duplicate, I likely have accidentally used the wrong namespace somewhere, but I have no way of identifying where.

This PR Fixes this by adding the namespace to the log output.

Does it fix an existing ticket?

No #000

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• tests are included and pass: yarn test (see details here) (no tests relevant)
• documentation is changed or added

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 37.50000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 95.16%. Comparing base (398b9ce) to head (3252e5e).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/transform.js	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1091      +/-   ##
==========================================
- Coverage   95.35%   95.16%   -0.20%     
==========================================
  Files          10       10              
  Lines        1918     1924       +6     
==========================================
+ Hits         1829     1831       +2     
- Misses         89       93       +4     

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[karellm]

Thanks!