Mail Rule "Send full copy to" and original intended recipient to receive single mail.

If one user name is in "To" field and also another user in "To" field have mail forwarding rule to first user then that first user will receive two mails, one mail received due to his name in "To" field and second mail received due the another user having mail forwarding rule for that first user.

Example : User A name in "To" field and also user B name in "To" field. User B have mail forwarding rule such that any mails received by user B will be forwarded to user A. So If any one send mail having user A and user B as recipient in that mail, then user A will received two mails. One mail received due to user A in recipient list and another mail received due to user B having forwarding rule for user A.

Hope that Domino would know if same recipient is in the email so that duplicate mail won't received.