That is a very good question. I have not tried (not much, anyway) to get the potmail or IMAP working properly but my few attempts did not even acquire mail at all. My previous experience with POPmail and IMAP is what led me to suspect that was your problem. There are two different things you can do. One of them is to select DOWNLOAD AND DELETE and then the only mail you have is the mail that is on your local machine in the Outlook files (.ost or .PST).
In that case, you will only get each email downloaded to you once and then the server will dump it. Another option is to delete all your local mail every time you pull mail from the server. That might sound absurd, but it actually makes sense if you are downloading mail to several different sites and only saving it at one site. From the site where you were saving mail, you copy The mail you want to keep off to the files where you store it. Delete everything else as you download it.
When I use an arrangement like that, I had all of the remote email readers DOWNLOAD AND SAVE. The site where I was saving email was set to DOWNLOAD AND DELETE. That way, when I had saved the email, it no longer appeared in the other male readers.
That entire discussion assumes you were using pop mail. IMAP handles that a little differently and I have never tried using that approach with IMAP. With IMAP, each male reader sees what is on the server. If you delete email in your inbox, it is deleted on the server as well. Obviously, you would want to save any important email before you deleted it from your inbox. (Moving mail from your inbox to the long-term folder makes sense because then it is automatically deleted from the inbox when you save it.)
I hope that my voice typing is sufficiently legible because I have not proofread my voice typing for this response to closely (because I am driving and should not be texting at all!).