how to have google apps engine send mail- not send a copy of the mail to the sender
You can't.  Sending email from someone without their knowledge isn't permitted by App Engine.
You can send email from any administrator address; you could add a "donotreply@yourapp.com" type address as an administrator and send email from that address.
I'm using GAE send mail- but I dont want the sender of the mail to get a coppy of the mail.
as for now, when a user is sending mail he gets a mail saying that he sent a mail to someone and the body of the sent mail, how do I disable that?