Are they leaving a copy of the messages on the server at all? We currently use qmail at work (in the process to switch to postfix cause well, its better) and it seems everytime a user who starts complaining about dup emails, its always their client, which is usually Outlook that is to blame, cause they're leaving messages on the server and its Outlook along with qmail that is at fault not remembering which emails they've downloaded or not, etc.
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