How do u prevent Mail from saving SENT emails LOCALLY?

I used Mail for all my Gmail accounts (IMAP) and for each account, Mail created a "folder" called Sent.

EVERY mails that I sent is kept locally in the Sent folder. Problem is, this create a duplicate because now I have TWO copies of the same sent email:

"Sent" Folder
Gmail's "Sent Mail" Folder

Any idea how to prevent Mail from keeping this sent mail locally? It's already sync'ed by the Gmail IMAP folder anyway.

 

Here's a possible work around/fix:
Go to Mail preferences>Accounts tab>select your gmail account>mailbox behaviors tab>in sent section you can choose to delete messages either everyday or as soon as you quit mail.
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OK, I saw that settings and change it to delete sent mesages when "One day Old". Will this also delete the sent message in Gmail server?

Also, does this means there is no way to change Mail from creating local copies of the Sent messages?

PS: I tried Entourage before and it prevent the above from happening, but it create havoc with syncing Calendars and Contact to my iPhone due to iSync issues in syncing Entourage <-> iSync <-> Calendard / Address Book <-> itunes.