Assalam o alaikum,can 1 person perform salat al janaza of 1 person 2 times?if no then whats the reason behind it?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If a person performs a funeral prayer on a dead person, then he should not perform the funeral prayer on him for a second time. The reason is that this act is not permissible.

Ibn Taymiyyah said: "According to the consensus of the scholars it is not permissible for a person who has already performed a funeral prayer on a dead person to perform another funeral prayer on him without a sound reason."

Indeed, many the scholars stated that it is disliked for a person who has already performed a funeral prayer to repeat the funeral prayer without a valid reason. However, if he repeats it, the prayer is valid and it is considered from him as an optional funeral prayer.

Performing the funeral prayer again for a valid reason is permissible, like if a person performs the prayer without ablution and then he realizes that he performed the prayer without ablution, then it is permissible for him to redo it.