Hi @anish.subedi,
If you are wanting to create new users with the create script, you will need to include a unique email for the user as well. If you have import mode off and Require Username enabled, the get_user script will actually only use the email. We have a backlog item in to address that the get_user script does not run again with a username, though we do not have an ETA for that at the moment. The workaround would be to use the get_user script to validate existing emails, but would only validate for existing username in the create script logic .
I also wanted to share this link to another community post that covers how your Custom DB scripts to be able to handle the email parameter being passed as either an email or a username: Username does not work for custom mongodb connection