When is competition allowed and when does it violate “Hasagas Gevul”?

Can a second business open up next to another and compete?
What if the second business will hurt or destroy the first business?
What does a Beis Din do when potential Hasagas Gevul issues come before it?
How does Hasagas Gevul apply in our modern, large cities nowadays?
Is the prohibition of Hasagas Gevul only on the new competitor, or also on those who purchase from him?