Duplicate Mails in Queue

Currently I'm struggeling with the following problem:

1. I Create a mail and send it to an user outside of the organisation.
2. This e-mail will be sent in CC to a queue in CRM.
3. This e-mail shows up in the queue.
4. If this is also tracked by the user, it appears a second time in the queue.
5. As a separate mail with a new id.

This only happens for one specific user in CRM.

Is there a setting, where this can be configured?