It can be frustrating when the same email shows up multiple times in your mail client. This article goes over two common causes of this and how to address them.
Corrupted POP3 profile
The most common cause of this problem is that you’re using the POP3 protocol in your email client.
The POP3 protocol is notorious for downloading the same message several times if you have the "leave a copy on the server" option enabled in your email client, especially if you're accessing the same email account on multiple computers.
If that's the case, the duplicate messages have the same message ID. You can view the message ID in the headers of the message. Here's an example of a message ID:
Unfortunately, there's not much you can do to eliminate duplicate messages unless you change the way you access that particular email account. It’s recommended that you only use IMAP to connect to your email.
View the POP3 vs IMAP article for further details.
Multiple addresses forwarding to one address
Another very common cause of this issue is that multiple addresses are forwarding to one address: this means that if the same message gets sent to more than one of the addresses being forwarded, they are all still delivered, though they look to be different email messages to the mail client.