FB.init has already been called

From the moment you pass parameters to the js.src like #xfbml=1&appId=X, FB SDK will auto init itself and thus FB.init will try to reinit..
So in your code, you don’t have to remove the FB.init function, just make sure you don’t pass parameters in the code that loads asynchronously the JS SDK.

Replace this:

js.src = "//connect.facebook.net/en_US/sdk.js#xfbml=1&appId=X";

With :

js.src = "//connect.facebook.net/en_US/sdk.js";