Hey there, NM. You DO NOT get another copy of the game if you buy it from here. They are different stores. Hope that clears things up!
In the past, when I found a game like this on a key-seller site, the key from there could be activated on steam. And the games are not visible in the steam store for me.
As long as the game on itch is complete and there are no differences to the steam release, I could buy it here. Steam would be more convienient for me.
Thank you