`/assign` quick command fails if user is already assigned

Summary

/assign quick command fails if user is already assigned

Steps to reproduce

1. Create MR, assing it to yourself and someone else
2. try:

     /unassign me
     /assign @someoneelse

Example Project

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31409

try:

/unassign @vshushlin 
/assign @jagood 

What is the current bug behavior?

Relevant logs and/or screenshots

https://sentry.gitlab.net/gitlab/gitlabcom/issues/850697/?query=is:unresolved

Output of checks

This bug happens on GitLab.com

Possible fixes

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