Sometimes, when a site-admin adds a new Atlassian Account to the instance, a portal-only customer account already exists on that email but the "Migrate to Atlassian account" process is not followed.

Suggestion

Adding a check for existing portal-only accounts on that same email would prevent duplicate accounts from being created which can cause several issues administering Jira.

Or even, run the process automatically should the customer account exist in the instance.