I am occasionally seeing duplicated files in my courier-IMAP folders. Here's an
example:

$ ls -nls 1260094560.39840_0* 6 -rw------- 2 1004 1005 4780 Dec 6 03:16
1260094560.39840_0.mail.covisp.net:2, 6 -rw------- 2 1004 1005 4780 Dec 6 03:16
1260094560.39840_0.mail.covisp.net:2,S $ diff 1260094560.39840_0.mail.covisp.net\:2\,
1260094560.39840_0.mail.covisp.net\:2\,S $

looking at the content of the file it is the exact same email, but looking at my
MUA the message only exists once.

Any ideas why this happens? Can I prevent it?

(the reason I want to prevent it is that I am scanning the maildir for messages
that are not marked as 'seen' with a terminal 'S'. I'm not doing anything that
would 'hold those messages, just doing a

ls −1dA $HOME/Maildir/.*/{cur,new}/*{net,','} 2>/dev/null | awk -F/ '{print $5}'

to count any unread messages)