Mail

Is it a IMAP or POP email account? You can check by going to Mail > Settings > Accounts. The type will be listed under the account. I would guess it is IMAP, and storing the sent message on their server as well as locally in the sent folder. Your sent folder then is seeing the copy from their server and your local copy.

There are settings at that same location from above in the Mailbox Behaviors and Server Settings that may account for the duplication.

Is your email an outlook or msn email and also a IMAP account. You may be better off deleting the account and adding it again, but this time choose Microsoft Exchange as the type of account. If the results are the same or you used Microsoft Exchange last time when you set it up, try adding it as an Other Mail Account.

Are you using Apple Mail?

Do you use an extension called Antidote. You can check at Mail > Settings > Extensions? If it is there you can try to disable it.

Does the receiver see 2 emails or do you see 2 emails under the Sent Folder of Mail?

A340-300 wrote:

Another thing bothering me. In MAIL I get multiple unresponsive blank pages generated while using it. The only way to clear these pages is by quitting mail.

Verify you have no unsent/stuck mail in the Drafts or outbox folder— if so remove/delete those, and compare your results.

Quit Mail and relaunch if necessary