Is it permissible to repeat the funeral prayer if the Imam prayed at the feet of the deceased, believing it to be the head?

The prayer led by this Imam is valid, and there is no need to repeat it, because his standing at the head of the deceased is a Sunnah, not an obligation. As long as the deceased is present before him, wherever he stands is sufficient. It is Sunnah for the Imam to stand at the chest or head of a male deceased, and at the middle of a female deceased. It is not obligatory for the Imam to stand directly in line with the deceased, but it is recommended. If he does not do so, or stands too far away, it is makrooh (disliked). As for repeating the funeral prayer for one who has already prayed it, that is makrooh, but it is hoped that the Imam was excused due to his lack of knowledge regarding the ruling.