Rashi on Deuteronomy 7:23/רש"י על דברים ז׳:כ״ג

1והמם AND HE WILL CONFOUND THEM — This word is vowelled entirely (both syllables) with Kametz, because the last ם is not part of the root, and it is equivalent to וְהָם, “And he will confuse”, אותם “them”. But in (Isaiah 28:28) וְהָמַם גלגל עגלתו the verb המם consists entirely of root letters, therefore half of it (one syllable) has Kametz and half of it Patach, just like any other verb of three consonants.