Jesus’ name had become well known: The successful mission in Galilee and Perea by six teams of disciples helped spread news about him throughout the region (6:12–13).
John the Baptist has risen from the dead!: King Herod may have thought that John the Baptist had come back to life and was now working through Jesus. This idea is similar to how Elijah's spirit came upon Elisha.
Or, he might have been expressing frustration, saying something like, “This is John the Baptist all over again!” (the same kind of powerful prophet has appeared again).