I would agree with your interpretation if there were a glossary entry for effect, and if the entry you quoted stated that spells "have one Effect each". That would make Effect a specific game term like Study Total or Target or Warping, rather than just another English word with a variety of definitions and colloquial uses. (See the recent Aegis rewrite, which uses the word effect in three different ways in a single sentence!*)
As it is, I don't think your version is what was intended, even though I actually prefer it. I may HR it in the next saga I run as something like "the same spell cast by the same magus (or enchanted item) on the same target only causes Warping once in a given season".
*Said sentence being, "An effect that would, in the absence of the Aegis, have an effect within the area of the Aegis, must Penetrate the Aegis in order to take effect."
Of course I am. I was using it as an example of English in use, and of the fact that effect has quite a few meanings. Without a specific definition provided in the rules, and without a capitalized game term that calls attention to the special use of the word, I conclude that effect has no such special use. In contrast, it is clear that Warping does not refer to e.g. moving a ship in a particular manner, whereas warping quite possibly might, depending entirely on context.