This happens because your are sending via the SMTP server of Gmail and have Outlook configured to save a copy of each Sent Item. As all the messages that you sent via the Gmail SMTP server also automatically store a copy in your Sent Items folder of the Gmail account, you'll end up with duplicates.

Why does my sent email show up twice?

The reason there are duplicates in the Sent Items folder for some email providers is that the server automatically creates an item in the Sent Items and Outlook uploads the same item again. For Read/Unread status of an email Outlook relies on the server. If the status from server is unread Outlook sets it accordingly.

Why are my sent emails showing twice on Iphone?

It is possible that you have the sent items saving on the server which will allow the Mail app to have it twice. This is because a local copy and the downloaded server copy is in the App.