One of my email accounts is sending multiple emails (Microsoft Outlook for Mac 2011)

Hello there,

I read other similar questions in the community, but nothing really applies to my situation..

First of all I am sending out emails from my computer, Mac, and I don't have any anti-virus blocking the way.
There are 5 email accounts on this Outlook (3 personal accounts and 2 info boxes).

This problem only applies to one of my personal accounts.

When I check it out on Gmail, I can also find duplicates there. 

I don't want people receiving the same email twice, which is what happens usually.

Can anyone help please?

Thanks