It doesn't copy itself, but when the original placement is free, the game places a new one there right away.

If you're played opus magnum; it's just like that. If you've played spacechem, consider it triggering a [slot] in command whenever the slot is free.

If you've played neither... just move the molecule forward a few ticks and you will see what happens :D

thanks for the tip!

I've just started and I literally can't get anything working. First tutorial level literally teaches nothing :/

I'm having that issue as well. Bonded a Cl to a C and moved the new molecule, but it left behind a new Cl which I can't get rid of. When I delete it it just instantly comes back, and when I move it, it creates another one :/