Identification:
Students are able to join (be candidate) to groups initially. When they are not accepted (removed as candidate) or removed from group (removed as member) they are able to see the group again on their group page. When a candidate request is made to a group they are removed before an exception is thrown.

Steps to reproduce:

1. Join to any group as a student
2. Accept the student to the group and then remove or directly remove without accepting
3. Try to join same group as same student

Error message:

The server endure the following error: PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '2-1' for key 'PRIMARY' in D:\Programs\XAMPP\htdocs\final-project\model\UserGroupDao.php:62 Stack trace: #0 D:\Programs\XAMPP\htdocs\final-project\model\UserGroupDao.php(62): PDOStatement->execute() #1 D:\Programs\XAMPP\htdocs\final-project\pages\group_controller.php(73): UserGroupDao::join('2', 1) #2 {main}