Duplicate emails are being created when two separate users have the same email address with different letter cases, even if an email is generated from a different username. For example, one email address begins with an upper case character, while the second email address begins with a lower case character with the same string from the first email address.
This only happens when an email notification has Event Parm 1 selected. If the notification does not have this field selected, the issue is not reproducible.
Give each user an individual email address. For example, use firstname.lastname@example.org only for one user and remove Example@example.com for another user. This removes duplicate emails and resolves the issue.
Related Problem: PRB585971