[BUG] Duplicates of users get saved

[mpgxvii]

Please check if the bug has already been reported in the Issues Tab and mention any related issues here.

Describe the bug

• When the task schedules for all users are generated, an IllegalStateException is getting thrown because of duplicate keys (key is subjectId) in the Subject->Protocol map (which is used to store the questionnaire protocol for each subject)
• This means that there are multiple users with the same subject ID which shouldn't be allowed
• When a user/subject gets added to the appserver, there is a check for whether the user with the same subject_id project_id already exists. If it exists an error gets thrown, otherwise it gets successfully saved
• There seems to be a race condition that allows the user to be saved even if the subject_id : project_id combination is already present

Priority
2

Difficulty
4

Possible Solutions

• Add unique constraint to users table
• Add unique constraint to User entity
• Fix the checking of existing user