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/activities/notification.go

@@ -151,7 +151,7 @@ func CreateRepoTransferNotification(ctx context.Context, doer, newOwner *user_mo
 			}
 			for i := range users {
 				notify = append(notify, &Notification{
-					UserID:    users[i].ID,
+					UserID:    i,
 					RepoID:    repo.ID,
 					Status:    NotificationStatusUnread,
 					UpdatedBy: doer.ID,