In Menachos 17b, the gemara explains a drasha:
ור"א א"כ לכתוב רחמנא אם האכל האכל א"נ אם יאכל יאכל מאי האכל יאכל שמעת מינה תרתי
And what would Rabbi Eliezer respond? He would say that if that were so, that the verse intends to teach only that halakha, let the Merciful One write either: If he’akhol he’akhol, or: If ye’akhel ye’akhel, repeating the same form of the word twice. What is the reason that the verse states “he’akhol ye’akhel,” employing both repetition and variation? Learn from this formulation two halakhot.
מאי האכל יאכל - מדשני קרא בדיבורא. שמעת מינה תרתי - למחשבת הקטרה בכזית ומחשבין מאכילה לאכילה
I've seen this setup in a number of gemaras (see also verb + its infinitive). Here Rashi says that the drasha, like any drasha, is based on a "שינוי": It could have just doubled the verb directly, so from the fact that the two verbs have different forms we can learn a drasha.
I don't know much grammar, but isn't this difficult? Am I wrong in thinking that there are hundreds of these doubled verbs in Tanach, and every single one has the same structure: first an infinitive form, then the regular verb. I think. If so, this is not the שינוי; doing it the other way with just a straight repetition would be a שינוי and a unique one.
Does someone explain this, or is my understanding incorrect?