I noticed I was getting multiple notifications when getting a new email in my GMail account. I'd see both the "New GMail email" notification icon, and the "New Email" notification icon. Swiping down the notification showed two entries: "New Gmail EMail (1)" and "New Email (GMail) (1)".
The reason for this is pretty simple. When you setup the phone it will automatically setup your GMail account into the GMail app. But a lot of people like the combined inbox and features of the Email app. I added my GMail to the Email app (having never realized the GMail app had my email setup already), thus getting two notifications.
I removed the account from the GMail app, and now I only get one notification.
I don't know if not having your GMail account in the GMail app has any ramifications to phone functionality, but so far so good!