Why am I getting multiple Gmail Notifications?
It sounds like maybe you added your GMail account to the regular Mail app.  So you are probably receiving notifications from both apps.

If you are only using the "GMail" app then find the "Mail" (not "GMail") app in your app drawer and run it and see if your mail is in there too.  If it is just delete the account from there and then you should only get notifications from GMail not Mail.