Why did the angel call Avraham's name twice?
Rashi: Calling someone twice by name is a sign of love. 1
Chazon Yechezkel: Once someone decided how to act, it is very hard to change. Therefore, the command to refrain had to be said more forcefully.
K'li Yakar: Because Avraham was concentrating so intensely that he did not hear the angels the first time.