I was receiving multiple notifications sent to my GMAIL IMAP account. I noticed that when I opened the first notification I was shown mail with the "all mail" label selected. Opening the second notification displayed the "important" lablel.
To resolve this issue, I logged into the gmail web interface, opened settings, clicked on labels. I then unchecked the "show in IMAP" box for the "all mail" label and the "important" label.
Perhaps this will help you.