Ubivox has been technically built to ensure that a receiver may only appear once on each list. If a receiver receives the newsletter twice then it is not because the same email address appears twice on the list.
However, this happens when a receiver, by mistake, has signed up to the list with two different email addresses. Thus the solution is to unsubscribe one of the two addresses.
You may ask the receiver to click the unsubscribe link in of the two emails that he/she receives each day, or you may ask the receiver to check which two addresses the newsletter is received at and that he/she unsubscribes from one of them.