Simple Q&A D&D 3.5 (by RAW) XXI

[The Dark Fiddler]

Q 745 If a wizard casts Ray of Stupidity twice on the same target in two consecutive rounds, do both spells take effect normally, or is there some rule that stops them from stacking?

A 745 They would stack. Ray of Stupidity deals ability damage, which means that the effect is cumulative; if, instead, it applied a penalty (as touch of idiocy does) they would not stack, because penalties from the same source don't stack.