Add warning on concurrent updates

[WhyNotHugo]

See: #309

[codecov]

Codecov Report

Merging #318 (234e372) into master (3eaf3aa) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #318   +/-   ##
=======================================
  Coverage   80.02%   80.02%           
=======================================
  Files          29       29           
  Lines        1857     1857           
  Branches      270      270           
=======================================
  Hits         1486     1486           
  Misses        259      259           
  Partials      112      112           

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