Don't return duplicated users who can create org repo (#22560) (#22562)

- Backport of #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 as a set to get distinct orgs.

models/activities/notification.go

@@ -157,7 +157,7 @@ func CreateRepoTransferNotification(doer, newOwner *user_model.User, repo *repo_
 		}
 		for i := range users {
 			notify = append(notify, &Notification{
-				UserID:    users[i].ID,
+				UserID:    i,
 				RepoID:    repo.ID,
 				Status:    NotificationStatusUnread,
 				UpdatedBy: doer.ID,