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Got the same problem yesterday and it was the mail app. It would fetch every hour and vibrate shortly (not the normal long vibration of a notification). In iOS 6, just go to settings > notifications > mail > select the email inbox (gmail, yahoo, et cetera) > scroll down to new mail sound > scroll up to vibration and select none. All fixed!
 
Don't think I've ever heard of a browser on an iPhone creating some sort of notification sounds/vibrations for a page in that browser.

well the mathematics is easy. i have got 2 vibrations and 2 notifications in fb.
 
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