In Dark Souls 2 I believe you can respawn bosses with a Bonfire Ascetic. This made me wonder if you respawn them and kill them again, do you get the Boss Soul again?
Yes. Ascetics respawn bosses and they will drop their boss soul. (At one point this was a speed run technique)
They can also drop items from higher playthroughs. For example, you can use an ascetic in your first playthrough on the Skeleton Lord's bonfire in order to defeat them again and get the clear bluestone ring +2, which is normally only available in NG+
In fact, one of the most efficient soul farming strategies is to use ascetics to respawn and fight the Giant Lord.
Also, bonfire intensity caps at 99, but you can still burn ascetics past 99 to respawn the boss