If you have a Windows Phone (I have Lumia 920 currently running 8.1.1 — and loving it! to quote Maxwell Smart) then you may have Gmail SMTP sync issues.
This problem plays itself out when you delete an email in the mail app on your phone and then later on see the same email in your inbox in Gmail.
If this is a problem for you, this is how I fixed it. Log in to your Gmail account, go to Settings –> Forwarding and POP/IMAP and set the radio buttons as follows: