Here are the details, from a post on the Facebook developer blog:
Earlier this fall, we announced the built-in like action, which allows you to publish like stories via the open graph. When publishing a like from your app, there are scenarios where notifications might be duplicated between your app and Facebook. For example, if you build an app where one Facebook user can like another Facebook user’s recipes, your app may want to send a push notification to the recipient of the like. However, if you publish the like to Facebook, we may also send a notification to that recipient. You can now set a parameter to suppress the Facebook notifications for published likes. See our documentation to learn more.
Readers: Have you ever seen more than one like on your news feeds from the same action?
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