Messages downloaded many times

Question

Mail keeps getting my messages multiple times. I can end up with 20 of the same email marked unread in the Inbox.
I need to leave them on the server so I can also retrieve on my MacBook.
I have 2 accounts - Different servers, same thing happens with both.
I did have this working and don't know what's up. Is it a pref or a server problem?

Answer 1

So what are your Preferences > Accounts > Advanced > Remove copy from server settings? How old are the messages that Mail keeps downloading repeatedly? That you need to leave the messages on the server for some time doesn’t mean you should leave them there indefinitely...
For POP accounts, Mail uses a file within the account folder to keep track of which messages have already been downloaded. That file may have become corrupt or the data stored there may have become invalid for some reason. You may try deleting that file so that Mail creates it anew, but that will also cause Mail to treat everything still on the server as if new and try to download it again:
1. Quit Mail.
2. In the Finder, go to the ~/Library/Mail/POP-username@mailserver/ account folder.
3. Locate one or two files named MessageUidsAlreadyDownloaded and move them to the Trash. Mail 1.x used MessageUidsAlreadyDownloaded to keep track of which messages had already been downloaded. Mail 2.x uses MessageUidsAlreadyDownloaded2 for the same purpose.
Alternatively, after having downloaded all the messages you want to have on both computers, you may try clicking Remove now in Preferences > Accounts > Advanced, which should both remove from the server the messages that Mail thinks have already been downloaded and reset MessageUidsAlreadyDownloaded.
Note: For those not familiarized with the ~/ notation, it refers to the user’s home folder. That is, ~/Library is the Library folder within the user’s home folder, i.e. /Users/username/Library.