- Add UNIQUE constraint on membership_application.user_id (one app per user)
- Add UNIQUE constraint on membership_fees.user_id (one fee record per user)
- Add validation checks in process_application.php before inserting
- Improve error messages for duplicate submission attempts
- Add migration script to clean up existing duplicates before constraints
- Update checkMembershipApplication to set session message on redirect
- Add comprehensive documentation of duplicate prevention architecture
Individual payments/EFTs are tracked separately in payments table
