I thought you rolled initiative each round.
You certainly can
do it that way, but the example of spell dueling (of all things) on pages 100-102 suggests that the original roll is kept throughout the combat
Example of Spell Dueling: The combat consists of The Emerald
Sorcerer, his two emerald soldiers, his nemesis Magnus
the Gray, and Magnus’ apprentice Athle the Astounding.
Initiative rolls are as follows:
17 = The Emerald Sorcerer
15 = the two emerald soldiers
12 = Magnus the Gray
6 = Athle the Astounding
Round one. The Emerald Sorcerer casts magic missile. Magnus
immediately declares a counterspell of magic shield. Athle
the Astounding declines to counterspell. Even though the
emerald soldiers are next in initiative order, the spell duel
is resolved first.
• At Magnus’ initiative count, he can take no action because
he already counterspelled.
Round two. In the second round, The Emerald Sorcerer
launches another magic missile. In this round, both Magnus
the Gray and Athle the Astounding choose to counterspell.
Magnus casts magic shield again, while Athle casts magic
Plus, I've been lucky enough to get to play in games run by Joseph, Harley, Doug, & Dieter, and that's the way they did it.