The problem of duplicate users on members page and issue board

Summary

On the members list page, there are occasional duplicate users. The current MR is used to solve this problem.

I created a test group to demonstrate the problem: https://gitlab.com/groups/test-repeat-user/subgroup/-/group_members

Another page with the same problem is: https://gitlab.com/groups/test-repeat-user/subgroup/-/boards

Steps to reproduce

0. Prepare a rootgroup and subgroup
1. In the rootgroup, invite user through unregistered email
   
2. The above email will receive an invitation link, through this link (remember to change the browser), register a new user
   
3. In the subgroup, invite the new user again
   
4. You're done! On the members list page of the subgroup, you can see that this user appears repeatedly
   

Example Project

https://gitlab.com/groups/test-repeat-user/subgroup/-/group_members

What is the current bug behavior?

What is the expected correct behavior?

All member entries should be displayed, depending on how many times a member has been added to a group or project.

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