Email check not done while making an online Contribution as an authenticated Drupal user

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 - user1@email.com
user2 - user2@email.com

4 - Login as user1 and make a contribution using the contribution page in step 2
change prefilled email from user1@email.com to user2@email.com.

5 - Submit and Confirm.
There's a CiviCRM database error.