Yeah, in the world of BaDIK, same surnames can only be used by relatives.
Considering you can be called "Fuckface" and the game uniquely acknowledges it, why did DPC get all serious when it comes to a surname?
Sage: "Yeah? That would make you "Fuckface Fuckface". Stop fucking around."
mc :"Yeah it's so silly isn't it Sage, not like "Fuckface Smith", not silly at all.
"
It would have been better if the mc could say all those surnames, and Sage responded something like this:
Sage: "Tailor...
Aren't you two... fucking? So you've been giving it to your own sister?"
mc: "For fuck's sake Sage, not everyone with the same surname is related!"
Sage: "So you are fucking her then... good to know...
"
mc: "Ahh you sneaky bitch!
"
Sage: "Ahaha!
"
or
Sage: "Burke?! Really? So you've been fucking your mom and your sister?!
"
mc: "What?! No! I mean, you know I'm not related to you or your mom!"
Sage: "Hang on, you
have been fucking my mom?
"
mc: "Oh shit..."
And then we know
where that leads to.