Thanks Murphy. It worked for me too.
Issue:
When a student tried to make a submission, the student was presented with an error message:
"Updating user information failed because user email was changed to an address that is already associated with an account".
Fix:
1. Go to Site Administration > Plugins > Activity modules > Manage activities
2. Find Turnitin Assignment activity - click on Settings
3. Now click on Unlink Users (screenshot below)
4. Search for the user(s) and click on Relink Users button