If I buy a game, and my friend buys a game, we can both own a copy of the game, without owning the same copy. My disc of Rollercoaster Tycoon 2 is not the same

If I buy a game, and my friend buys a game, we can both own a copy of the game, without owning the same copy. My disc of Rollercoaster Tycoon 2 is not the same as my friend's disc of Rollercoaster Tycoon 2