If I hit you with Ray of Frost, and my friend hits you with Ray of Frost immediately after, do you have your speed reduced by (10x2=)20 until the start of my next turn?
Or do you only lose 10 speed since the effect does not stack?
(Note: this is also applicable with the Warlock invocation Lance of Lethargy)
From the SRD:
Combining Magical Effects
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.
For example, if two clerics cast bless on the same target, that character gains the spell’s benefit only once; he or she doesn’t get to roll two bonus dice.
Although Ray of Frost's damage would apply with each hit, the speed reduction is an effect that the above rule would prevent from stacking with itself. However, it would still stack with other speed reducers like Lance of Lethargy or Frost Lance.
The Basic Rules state:
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap.
Ray of Frost would not behave differently to require a specific ruling over the general rule above because there is nothing in it's language stating it would.
