If you're using an IMAP client like Mac Mail to manage your email, you must also install Google's Advanced IMAP Controls Lab. It enables you to choose which GMail labels show up in your mail client and prevents multiple copies of the same email from being downloaded.

1. Log into GMail
2. Click the Settings link or the gear icon at the top left of the screen and open the Mail Settings option from the drop-down menu
3. Select the Labs tab
4. Scroll down to the Advanced IMAP Controls lab (the list is in alphabetical order)
5. Click the Enable button
   

   

6. Scroll to the top or bottom of the list and click Save Changes
   

   

7. Open the Settings screen again and click the Labels tab
   

   

8. Next to each of your labels, you'll see a new checkbox with the label Show in IMAP
   

   

9. Next to All Mail, uncheck Show in IMAP. This will prevent your email client from downloading duplicate messages
   

   

10. Log out of GMail.