multiple emails to the same person

We have one user that is getting multiple emails for everything that
has been sent him. I have currently changed the settings on his
Outlook 2003 so that emails are not saved on the server and this seems
to have solved the problem. Is there anything else I can do? Is there
some diagnostic that would tell me this is happening? What causes this
to happen? How can I figure out the cause?

I do need to reinstate saving on the server at some point.
postfix-2.2.10 mail saved in /var/spool/mail