When a user logs into Drupal and makes an online contribution, it breaks down if it selects an existing email id from the civicrm_email table.
1 - Create a Payjunction based processor
2 - Create a contribution page using the payment processor from step 1
3 - Create users in drupal
user1 - email@example.com
user2 - firstname.lastname@example.org
4 - Login as user1 and make a contribution using the contribution page in step 2
change prefilled email from email@example.com to firstname.lastname@example.org.
5 - Submit and Confirm.
There's a CiviCRM database error.