Bad User mail address-- Outlook

Let's see how i will explain this to u.

First of all we are running Exchange 2007 and some user have Outlook 2003 and others Outlook 2007.

When we try to send a new mail message to user which name is Roger, by entering a "R", we get a list with all names that begins with a "R", but the user Roger appears 3 times in this list, which only 1 we are able to send email to, the other 2 we receive a msg back that the user is not recognized.

So my question is, how can i remove these other two faulty "Roger" from the list?