MarkB said:
Then wouldn't multiple castings simply overlap and extend the duration?
Much like casting 3E Shield twice didn't result in two disks of force?
With the first casting, you become a creature under the effects of Cloud of Knives. With the second casting, you are still a creature under the effects of Cloud of Knives... it's just harder for a Dispel Magic to make you
not a creature under the effects of Cloud of Knives.
I have a couple of quibbles about some of the wording, but the general theme of the answer from the 3E Main FAQ seems reasonably on track:
Can you cast two shield spells and cover yourself against
attacks from anywhere on the battlefield?
No. The spell’s target is you (the caster), so when you cast it
you get a disc of force you can use for defense. If you cast the
spell a second time, you still have a (single) disc of force you
can use for defense. If the spell’s target was the disc (or if the
spell was an area of effect spell) you could have more than one,
just like you can summon more than one monster or create
more than one wall of force. The target is you, however, so you
can have only one shield spell running at a time.
The closest fit in the core 3.5 rules:
Same Effect More than Once in Different Strengths: In cases when two or more identical spells are operating in the same area or on the same target, but at different strengths, only the best one applies.
So if we had a CL 11 Cloud of Knives and a CL 10 Cloud of Knives on the same person, only the CL 11 would apply.
If both are CL 11, then this rule is not an exact fit - they are not 'at different strengths'... but presumably the same principle (as illustrated by the 3E Shield answer) holds. (One could, perhaps, argue that two otherwise identical Cloud of Knives spells with different remaining durations are operating at different strengths...?)
-Hyp.