Why the stored memory is used 2 times in a single layer?

[kazeto111]

Hi MemNN authors (@tesatory) !

I read the paper "End-To-End Memory Networks", and I have 1 question.
Why you use the stored memory x1 , .., xi 2 times in a single layer like figure1? (I attached the figure just in case

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Why that is not 1, or 3 or more times? Is there any Mathematical reason or just rule of thumb?