In a tactical battle with two spiders, one of the spiders webbed my units successfully. Then, immediately after that, the second spider tried to web the same unit (again successfully) within the same turn. I am guessing either:
1) The spell effects are checked at the beginning of each turn so the second spider doesn't "see" that the target unit is already webbed.
2) The spell effect of "web" is buggy and it is not seen by the targeting AI.
I remember this being an old problem, it still exists in 1.7. I don't know about other spell effects.