duplicate sent email

I suspect you are using an IMAP account and when you send an email it is saved locally and sent to the mail server which put it into its Sent folder and sends it back to you in your Sent folder. If using Mail, check Mail > Settings > Accounts and select your account and choose "Mailbox Behaviors". What do you have under Sent Mailbox.

If not Mail, let us know what you are using and type of Email account.

Go to Mail > Settings > Accounts. Select your Mail box and choose "Mailbox Behaviors". What do you have under "Sent Mailbox"?

Are you using Apple mail or MS Outlook?