On JIRA Service Desk, sending an email to the mail handler and another customer using the TO field, results in the user being added as a participant.

On documentation: Adding people to participate in requests it states:

Note that email recipients in the TO and BCC fields will not be added as request participants.

Problem:
If JIRA Service Desk has as a Customer a user with the same email address than the one being used to fetch emails, it results in that user being added as Participant for every issue created, and thus receiving notifications for everything and processed again by JIRA.
JIRA Service Desk rejects incoming emails from itself to prevent loop.

Steps to reproduce:

1. enable a Service Desk incoming mail
2. create a user with that same email address (could be by sending an email from it's own address if create users is enabled)
3. verify the user is a Customer of the Service Desk and can be added as Participant
4. Send an email to the Service Desk
   Result: User with the same email as the service desk gets added as Participant
   Expected: an issue is created and the User with the same email address is not added as participant.

Workaround:
If the user with the same email address is not being used, disable it from the User Management to prevent him being added as a Participant, or remove it from the group of Customers of the Service Desk.