This also happens with (quite common) high templar vs ghost duel. Both have ability to instantly kill the other type, with the same range. When you move them fu

This also happens with (quite common) high templar vs ghost duel. Both have ability to instantly kill the other type, with the same range. When you move them further than that range, and order both to use that ability on each other - the unit that was produced earlier always wins. It's not game breaking, but players noticed this. To avoid such problem with marines shooting (this would be game-breaking, because marines make up bulk of terran armies for most of the game) - there is miniscule random factor added to delay between shots for each marine. If it was important enough - they would fix the high templar/ghost problem the same way. BTW in e-sport games (like Starcraft) - finding such bugs and abusing them creatively is part of game. For example mineral-walking is legitimate techinque to micromanage your workers, used even in low-level matches, and it's caused by workaround of bug in Starcraft 1.