When creating a new mail/thread in any of my email accounts (2 exchange accounts and a IMAP account) it creates and sends 2 of the email. On further inspection of the double emails, it seems like it is sending one from the email provider (John C. Doe, email@example.com) and one from the Mail client, which has a different name (John Doe, firstname.lastname@example.org). I have tried changing the name in the Mail software to match the name on the account, but it then sends two identical emails. I can change the name of the account, so I don't really care which one sends the email, but it is a regular issue. It doesn't do this from any of the other email clients on the app store, or if I send an email from online or on my phone, so it is an issue with Mail. It also does not send duplicate messages on a reply, forward, or other type of message.