There are two likely reasons for this one. One is that someone is sending you the same message over and over (this isn't very common). The other is that your mail client uses POP3 and has taken a dislike to one of the messages it is trying to fetch. What happens is something like this:
What the mail client should have done is to fetch each of the messages then tell the server to delete them (they are on the client machine now, so there's no need to keep them on the server any more). Since the deletion didn't happen, then the mail client connects again a little while later...
If you can remove the message that upsets your mail client everything will return to normal. To do this, you need to access your mailbox using a different mail client - if you have access to a webmail interface, that's probably the simplest way.
This one typically crops up with junk email and mailing lists. As explained in The Format Of a Mail Message the details of who a message is sent to is provided in the message envelope, which is not normally seen. When a message is sent to multiple people, they are all listed in the envelope, but only a few are usually mentioned in the message itself.
There isn't one, as this is just a misunderstanding, not an actual problem.