Fix the problem of duplicate users

What does this MR do and why?

Resolve #414557 (closed)

Related Jihu issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/2885

What

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

Why

When obtaining the member list, a scope named distinct_on_user_with_max_access_level will be used to deduplicate the members. Now it uses (user_id, invite_email) instead of (user_id) to deduplicate (because user_id may be nil).

However, after the operation in "How to set up and validate locally", the following types of data ⬇️ will exist in the database, resulting in deduplication failure:

SQL changes

Click to expand

Before

[5] pry(main)> GroupMember.distinct_on_user_with_max_access_level
  GroupMember Load (1.4ms)  SELECT "members".* FROM (SELECT DISTINCT ON (user_id, invite_email) * FROM "members" WHERE "members"."type" = 'GroupMember' AND "members"."source_type" = 'Namespace' ORDER BY user_id, invite_email, access_level DESC, expires_at DESC, created_at ASC) members WHERE "members"."type" = 'GroupMember' /*application:console,db_config_name:main,console_hostname:MacBook-Pro.local,console_username:deer,line:bin/rails:4:in `<main>'*/

After

[5] pry(main)> GroupMember.distinct_on_user_with_max_access_level
  GroupMember Load (1.7ms)  SELECT "members".* FROM (SELECT DISTINCT ON (COALESCE(user_id::Text, invite_email)) * FROM "members" WHERE "members"."type" = 'GroupMember' AND "members"."source_type" = 'Namespace' ORDER BY COALESCE(user_id::Text, invite_email), access_level DESC, expires_at DESC, created_at ASC) members WHERE "members"."type" = 'GroupMember' /*application:console,db_config_name:main,console_hostname:MacBook-Pro.local,console_username:deer,line:bin/rails:4:in `<main>'*/

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

This step is used to reproduce the problem:

Click to expand

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
   

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

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cc @mtan-gitlab @icbd @chaomao