[#826] - get() returned more than one Tag -- it returned 2!

[fazeelghafoor]

Fixes #826

• add check for MultipleObjectsReturned exception returning only the first tag when get() returns more than one
• add deduplicate_tags management command which keep the first tag and merge all duplicates into it (relationships), then delete the duplicates.
• add test for deduplicate_tags command

[fazeelghafoor]

@rtpg please review and let me know if any changes required.

[codecov]

Codecov Report

Attention: Patch coverage is 71.73913% with 13 lines in your changes missing coverage. Please review.

Project coverage is 90.95%. Comparing base (d0fe810) to head (5835dcb).
Report is 50 commits behind head on master.

Files with missing lines	Patch %	Lines
taggit/management/commands/deduplicate_tags.py	76.19%	9 Missing and 1 partial ⚠️
taggit/managers.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #914      +/-   ##
==========================================
+ Coverage   89.21%   90.95%   +1.73%     
==========================================
  Files           9       11       +2     
  Lines         779      851      +72     
  Branches      142      157      +15     
==========================================
+ Hits          695      774      +79     
+ Misses         62       52      -10     
- Partials       22       25       +3     

Flag	Coverage Δ
	90.95% <71.73%> (?)

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[rtpg]

Thanks for the PR! I made some changes to simplify the management command and make it explicit that the ordering is always by pk (though that's a bit spurious given the behavior of first).