Are you using Outlook Express as mail client, then check if you are able download all the mails which are on the server in one shot, if not every time you download mails multiple copies of mails will be downloaded.
-Next thing is to Check if there are any rules in user MS OUTLOOK (There may be hidden rules, means rules created earlier and deleted, many such rules will be some times never deleted), Try opening outlook with switch with ‘OUTLOOK /cleanclientrules’
-Is the user gettting multiple copies of e-mails for every mail ar any particular mail.
-Are sending mails to user alone or to any disteibution list.
Revert for any further queries.