API: Multiple same labels

Summary

Editing an issue with adding multiple time the same label add multiple time the same label

Steps to reproduce

• curl --request PUT --header "PRIVATE-TOKEN: xxxxxxxx" https://gitlab.com/api/v4/projects/project_id/issues/xxx?labels=yourlabel,yourlabel,yourlabel,yourlabel
• see that the label "yourlabel" has been added 4 time

Example Project

https://gitlab.com/antony360/labels-exemple-project/issues/2

What is the current bug behavior?

It register multiple time the same label

What is the expected correct behavior?

It should register the label only one time

Relevant logs and/or screenshots

(Paste any relevant logs - please use code blocks (```) to format console output, logs, and code as it's very hard to read otherwise.)

Output of checks

(If you are reporting a bug on GitLab.com, write: This bug happens on GitLab.com)

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

(If you can, link to the line of code that might be responsible for the problem)