When I get incoming email from my gmail account it shows as two identical emails grouped together. One is flagged as Folder: All Mail. How do I resolve this so Windows Phone recognizes this as one email?
Gmail uses labels like folder.
What is probably happen is that you are syncing all folders and your e-mail is shown in more than one label.
The problem is the email client app sees the "inbox" folder and "All Mail" as 2 different folders. So you have turn off the syncing the "All Mail".
Slide the menu on the bottom. Look for "folders" and select "show all Folders". Select "All Mail". Now if you slide the menu on the bottom, you will see an option "don't sync this folder". Hit that, and that should do it. Once it done syncing hit back and come back.
Now it will display the same emails only from inbox.