Why do I receive duplicate e-mail messages?

There could be at least two reasons for receiving duplicate e-mail messages (i.e. getting the same email address two or more times):

Option 1: You have set up the email account (mail@my-best-domain.com) in two different e-mail clients using a POP3 mail protocol. You have most probably checked the "Leave a copy on the server" option. Therefore, when you check your email from a different location/computer/e-mail client, the mail client downloads the emails and leaves a copy on the server each time you access the email account.
If this is the case, simply uncheck the "Leave a copy on the server" option in your e-mail client settings. Another alternative is to use the IMAP protocol (IMAP generally keeps all the e-mail messages on the server, without downloading them).

Option 2: You have set up forwarding to Gmail or any other e-mail address, but on the other end another forwarding to the original mailbox has been set up, creating a loop. Each message received in your mailbox hosted here will be forwarded to the remote email account and then forwarded back and so on.