The reason this happens is because Gmail’s IMAP shows the same item in multiple folders, for instance when you’ve labeled an email it will show up in the label folder as well as All Mail and the Inbox, even though it’s the same email. Outlook can’t tell the difference.
The fix is to tell the To-Do bar to ignore any tasks that are in other folders other than All Mail.
Fixing Duplicate Flagged Items
Right-click anywhere in the To-Do pane’s task area, and then choose Filter from the menu:
Choose the Advanced tab, and then add a new rule by manually typing in the values into the fields:
Field: In Folder Value: All Mail
Make sure you type them in exactly (note that you can navigate the menu to find the “In Folder” item), and once you are done click on the Add to List button before you leave this screen.
Make sure that your list now contains the new item before you click the OK button:
Note: If you have more than one mail account and also flag messages in those, you will need to add an extra rule for including those folders as well. Alternatively you could block only certain folders using this method.
You’ll probably have to do a Send/Receive before the items disappear from the list.
You could also exclude items from All Mail by reversing the condition to exclude instead.
