A client called with a problem with her iPhone, MacBook and duplicate/old Comcast eMail. Now I don't usually do Macs, but I had some ideas to suggest, which worked! After learning that she had recently tried to activate the iCloud, I advised her to access her Comcast mailbox through an Internet browser (webmail) and to clean out old messages still in the Inbox.
The reason for this relates to the advanced settings entered when eMail is set up on a device (phone, pad, laptop, desktop). Typically, since multiple devices are involved, the default setting is to leave the messages on the "server" so multiple devices can pull a copy of the message. Otherwise, the first device would "move" it, and no other device would be able to find/download it. Those "remove after 14 days" and "remove when deleted" check boxes don't seem to work.
With eMail to a device, the actual process is to connect to the mail server, check for messages that the device has not yet copied, and copy the messages to the device. If the messages are not marked to be kept, then once all the new messages are copied, the device acknowledges it to the server and tells the server to delete.
When my client initiated the iCloud, somehow the indicators on the "old" messages in the Inbox were either reset, or the device is then treated as a new device, and needed to get all messages in the Inbox.
So the better precaution is set yourself a reminder to webmail into your POP mailboxes (not necessary with IMAP) on a monthly basis and clean out the old mail, and check your Junk/Spam folder too!