When publishing a Merge Request review sometimes comments are posted twice

Summary

This still requires some investigation but there's reports that sometimes when publishing a MR Review, comments are posted twice.

Please see:

1. !50059 (comment 467664757)
2. !50059 (comment 467665544)

Steps to reproduce

1. Start a review with at least one comment
2. ....unknown conditions....
3. Publish the review
4. Check that it posted same comment more than once (not always)

Example Project

1. !50059 (comment 467664757)
2. !50059 (comment 467665544)

What is the current bug behavior?

Sometimes, comments are posted twice.

What is the expected correct behavior?

Comments are posted only once.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes