Don't return duplicated users who can create org repo (#22560)

- Currently the function `GetUsersWhoCanCreateOrgRepo` uses a query that
is able to have duplicated users in the result, this is can happen under
the condition that a user is in team that either is the owner team or
has permission to create organization repositories.
- Add test code to simulate the above condition for user 3,
[`TestGetUsersWhoCanCreateOrgRepo`](a1fcb1cfb8/models/organization/org_test.go (L435))
is the test function that tests for this.
- The fix is quite trivial use a map keyed by user id in order to drop
duplicates.

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/fixtures/team.yml

@@ -140,3 +140,14 @@
   num_members: 1
   includes_all_repositories: false
   can_create_org_repo: false
+
+-
+  id: 14
+  org_id: 3
+  lower_name: teamcreaterepo
+  name: teamCreateRepo
+  authorize: 2 # write
+  num_repos: 0
+  num_members: 1
+  includes_all_repositories: false
+  can_create_org_repo: true

models/fixtures/team_user.yml

@@ -93,3 +93,9 @@
   org_id: 19
   team_id: 6
   uid: 31
+
+-
+  id: 17
+  org_id: 3
+  team_id: 14
+  uid: 2

models/fixtures/user.yml

@@ -104,7 +104,7 @@
   num_following: 0
   num_stars: 0
   num_repos: 3
-  num_teams: 4
+  num_teams: 5
   num_members: 3
   visibility: 0
   repo_admin_change_team_access: false