Hi Olly.
Thank you for the quick reply.
I am using mailgun to send and I am sending them to a gmail account. I have 5 gmail / google apps accounts and have never had a double sent or receive before. H
However I am not using mailgun for anything else, so I will try writing their support and see if they have any suggestions for a solution or perhaps are familiar with the problem.
However I also have google cloud print plugin in my WP-Pizza setup and as far as I can recall, when the email is double sent, the cloud print is also double printed. (I am not 100% on this but like 80% sure.)
Will write back when I have been in contact with Mailgun