There seems to be a little bug when using Gmail on the iphone - the problem being duplicate emails that are sent back into your inbox.
Well here's a handly little work around that will prevent you from receiving copies of the sent items in your inbox.
This fix really depends on how much you use the phone email over the browser email but anyway here goes:
To do this go into your iPhone settings.
Mail>Add Account > Other
select POP
Then enter your name, address, and the description you want for the email address
under Incoming Mail Server insert the following:
Host Name: pop.gmail.com
User Name:
yourname@gmail.com
Password: YourPassword
Outgoing Mail:
Host Name: smtp.gmail.com
User Name:
yourname@gmail.com
Password: YourPassword