No offense to all you guys' comments but don't you know that it's not unusually for completely unrelated people to have the same last name so they can just lie and say it is a coincidence
They could also explain they're cousins. I mean as far as I know marriage between cousins isn't illegal in Japan. I'm also talking 1st degree cousinry. Yes the genetic disorders are there but according to most I read online, it's been declining.
The reasonable response would be “I know, it’s such a strange coincidence, right? It’s like we’re already married!”, but I sincerely doubt that would happen.