It is important to remember that duas for the deceased are made in the janazah prayer itself. It has been observed that some Imams ask attendees to stay after the janaza prayer to perform congregational dua. There is no evidence to suggest that the Prophet Muhammad (peace be upon him), his Sahabah, or the Salaf ever did this. Any act that cannot be traced back to the Prophet (peace be upon him) or his Sahabah and is being considered as part of the Deen, is rejected and should be abandoned.