Assuming that you are using POP3, this can happen because of a couple of reasons.
1. There’s an email with a corrupt message header that your email client can’t download. As it downloads all your email, it gets to the message with the corrupt header and the download crashes. The next time your email client tries to download the email it starts over. When emails are downloaded, they aren’t deleted from the email server until after all the emails are downloaded.
2. There’s an email with a large attachment in your mailbox and your client is timing out when downloading it. Same basic idea as the other option. The client times out, and the next time it tries it starts by redownloading all your email that it already got.
You can contact your ISP and have them check your mailbox for a corrupt email, or a large email, and have them remove that email from your account.