>In each 7 years all atoms of our bodies changes completely. There is not a single atom in commom in your body today as in 7 years ago.
Not really true.
>So are you a different person?
No, because while some (or even if all) atoms in your body change, the cellular remains the same. This means, the information that defines you is still mostly the same. Your instincts are the same, your memories are the same and your visual appearance is the same.
>The kid died?
No the kid lives. That's how life is.
>If consciousness is just chemical reactions, how can it be constant if the atoms were changed?
It's because consciousness is not dependent on your atoms alone. It's dependent on the cellular and organic structure that the atoms form. It's like when you change the tire of your car. Even though the tire is different, it's still a car.
>If something clones every single atom of you and paste it by your side, the clone will be you?
It will be identical and people won't be able to tell the difference between the two of you. So for them it's you. But for you, it's a different person, which happens to have all your knowledge, experience and interests.
>Will you feel his existance, too?
Not more or less than you can feel the existence of any other person.
>Both of "you" will feel each other existance?
Both of you will be copies, just like twins. Nothing more.
Your existence is your consciousness. Your consciousness is your brain.
It's just as simple as that. Don't try to make it something magical. It's not and it will never be, no matter how much you want it.
I suggest you attend some neurology course. Very important, NOT psychology, neurology as the field of biology.
Here OP, watch this:
youtube.com/watch?v=wfYbgdo8e-8