Does an extra "Allahu Akbar" during the Imam's Witr prayer before the Qunut supplication invalidate the prayer, or is it permissible to follow him based on the principle "Whoever's prayer is valid for himself, it is valid for others"?

The takbir (pronouncement of "Allahu Akbar") before Du'a al-Qunut in the Witr prayer, according to the Hanafis, has no evidence from the Sunnah or qiyas (analogical reasoning). Imam Shafi'i considers it an extra addition. Nevertheless, the prayer is not invalidated by it, because it is a prescribed utterance in prayer, even if it is out of its proper place. As for following the Imam in it, by analogy with Imam Ahmad's view on extra takbirat in the funeral prayer, the Imam should not be followed in it because it is neither prescribed nor an addition.