Mail being received back to sender


there is something strange going on (at least for me). A client of mine uses outlook 2007 for sending and receiving mails. He reports that every mail he sends to anyone (on the same domain or outside) receives a copy of the mail in his account too. As if he was the recipient of the same mail. Whereas the mail is received on the other end to actual recipient without any issues.
I am not a user of of outlook or have some document knowledge of it, can anyone help explaining this please?
Thanks Dan,
I confirmed user about any unusual outlook settings. Apparently all is good at his end. where to look for this in cPanel?

As I said I don't think that it can be done at the user level, only at the root level as it would require modifying the Exim configuration.

Is the user connecting to his account using IMAP? If so it could be that he has inadvertently set the sent items to go to the inbox instead of sent items. If he is using IMAP have him take a look here to configure it.
I probably did not check that. I will ask him to confirm me the IMAP/POP settings and also take guidance from this article/screenshot you shared.