Many, many times in Tanach, including Chumash, a verb has its infinitive nearby. Examples include B'reshis 2:16 מִכֹּל עֵץ הַגָּן אָכֹל תֹּאכֵל (I think that's the first example in Chumash) and Bamidbar 16:13 כִּי תִשְׂתָּרֵר עָלֵינוּ גַּם הִשְׂתָּרֵר. This is generally translated as providing emphasis; thus, for example, R' Kaplan's chumash translates B'reshis 2:16 as "You may definitely eat", and the JPS chumash translates Bamidbar 16:13 as "but thou must needs make thyself also a prince over us". (FWIW Christian translators, l'havdil, usually translate it similarly.) IIRC when I learned Chumash in grade school, the repetition was translated with an added "surely".
My question is, does anyone have a written (Jewish) source for such interpretation of the repetition (or for any other interpretation of it), especially among the rishonim?
Note that my question is about the p'shat of such p'sukim, not about d'rashos on them.