If a slaughterer mentioned another name along with the Name of Allah when slaughtering an animal, would that animal be lawful to be eaten? For example, someone said, “In the name of Allah, in the name of the Messenger.”

It is not permissible to mention other than the Name of Almighty Allah when slaughtering an animal. But is this slaughter valid or not? If he did not intend with this word to associate others with Almighty Allah, but it was just to seek blessings, the animal he slaughtered would be lawful.
Nevertheless, in this case we should advise him not to add any other name to the Name of Almighty Allah. Almighty Allah says, “Thus you shall not eat of that over which the name of Allah has not been mentioned.” However, if he did not intend to associate others with Allah, we cannot go to the extent of deeming the animal he slaughtered as unlawful. Allah knows best.