Secret underground market on Ellen Street

If you buy a copy you get 2 extra copys. Each of that extra copys give 1 extra copy. Also: it was in a bundle.

everybody who buys the ship gets 2 more copies of the game

Lots of The Ship from the Steam Christmas event and from a bundle.
Some copies would "spawn" more copies in the Steam inventory after you added them to your library (see the Steam forum page for more information)