Mark 5:1-2 “And they came over unto the other side of the sea, into the country of the Gadarenes.
And when he was come out of the ship, immediately there met him out of the tombs a man with an unclean spirit,”
And when he was come out of the ship, immediately there met him out of the tombs a man with an unclean spirit,”
Matthew mentions two demoniacs, whereas Mark and Luke mention only one of them. This is no contradiction. If there were two (and there were!), then there was also the one that Mark and Luke mention. Mark and Luke mention the one who was pertinent to their story.