[Open311] Check for identical latest update.

[dracos]

This already existed for Merton, but it seemed like a good thing to have in general - could help with e.g. Bucks duplicate updates on occasion (though we've hidden some status codes there as a different solution).

[codecov]

Codecov Report

Merging #4007 (1a4c773) into master (1a4c773) will not change coverage.
The diff coverage is n/a.

❗ Current head 1a4c773 differs from pull request most recent head b1ea62b. Consider uploading reports for the commit b1ea62b to get more accurate results

@@           Coverage Diff           @@
##           master    #4007   +/-   ##
=======================================
  Coverage   82.68%   82.68%           
=======================================
  Files         361      361           
  Lines       25017    25017           
  Branches     3763     3763           
=======================================
  Hits        20685    20685           
  Misses       3146     3146           
  Partials     1186     1186           

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[chrismytton]

♊