Messages arriving in Mail from my ISP keep arriving in duplicate. Why?
POP
Why does Mail tell me that there are incoming messages, but then they do not appear in a mailbox?
Most common problems are failure to collect or send, but this gives you clues to solving many other problems such as stuck messages.
Electronic mail remains one of the fundamental network services. Here I explain POP, IMAP, and SMTP basics.
The Eclectic Light Company 