Adding new tags during node update can seem to result in duplicate tag-node relation creation

[sentry]

Sentry Issue: STUDIO-E2T

IntegrityError: duplicate key value violates unique constraint "contentcuration_contentnode_tags_contentnode_id_cfa67da1_uniq"
DETAIL:  Key (contentnode_id, contenttag_id)=(93cc2b88b63c42a0a1a163a3bb6c74d6, 35af2d4597c348b5b210980a97c0e1ee) already exists.

  File "django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "newrelic/hooks/database_psycopg2.py", line 51, in execute
    **kwargs)
  File "newrelic/hooks/database_dbapi2.py", line 25, in execute
    *args, **kwargs)
  File "django_prometheus/db/common.py", line 68, in execute
    return super(CursorWrapper, self).execute(*args, **kwargs)

IntegrityError: duplicate key value violates unique constraint "contentcuration_contentnode_tags_contentnode_id_cfa67da1_uniq"
DETAIL:  Key (contentnode_id, contenttag_id)=(93cc2b88b63c42a0a1a163a3bb6c74d6, 35af2d4597c348b5b210980a97c0e1ee) already exists.

(8 additional frame(s) were not displayed)
...
  File "django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "newrelic/hooks/database_psycopg2.py", line 51, in execute
    **kwargs)
  File "newrelic/hooks/database_dbapi2.py", line 25, in execute
    *args, **kwargs)
  File "django_prometheus/db/common.py", line 68, in execute
    return super(CursorWrapper, self).execute(*args, **kwargs)

IntegrityError: duplicate key value violates unique constraint "contentcuration_contentnode_tags_contentnode_id_cfa67da1_uniq"
DETAIL:  Key (contentnode_id, contenttag_id)=(93cc2b88b63c42a0a1a163a3bb6c74d6, 35af2d4597c348b5b210980a97c0e1ee) already exists.

  File "django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "newrelic/hooks/database_psycopg2.py", line 51, in execute
    **kwargs)
  File "newrelic/hooks/database_dbapi2.py", line 25, in execute
    *args, **kwargs)
  File "django_prometheus/db/common.py", line 68, in execute
    return super(CursorWrapper, self).execute(*args, **kwargs)

IntegrityError: duplicate key value violates unique constraint "contentcuration_contentnode_tags_contentnode_id_cfa67da1_uniq"
DETAIL:  Key (contentnode_id, contenttag_id)=(93cc2b88b63c42a0a1a163a3bb6c74d6, 35af2d4597c348b5b210980a97c0e1ee) already exists.

(16 additional frame(s) were not displayed)
...
  File "django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "django/db/backends/utils.py", line 64, in execute
    return self.cursor.execute(sql, params)
  File "newrelic/hooks/database_psycopg2.py", line 51, in execute
    **kwargs)
  File "newrelic/hooks/database_dbapi2.py", line 25, in execute
    *args, **kwargs)
  File "django_prometheus/db/common.py", line 68, in execute
    return super(CursorWrapper, self).execute(*args, **kwargs)

[rtibbles]

Investigation of this suggests that this occurs if two sync requests with the same tag adding operation are happening in parallel, but evidence is not yet conclusive.

[rtibbles]

I tried adding a simple unit test for this previously, but was unable to replicate the issue locally. It's possible that I missed a trick - some of the recent reports on Sentry had multiple tags being updated at once, so that might help to cause the issue?