You wouldn't respond to the person because they see it as trust and then they will message you more
hello do i know you?
you could look through the persons friend list to see if they know anyone that you know including family
if you find friends or family that you know you could ask them how and where they know them from
where do you know my friend from
you could ask your family or friends if they know the person who is messaging me
if your friends or family dont know the person then you can block them and you could tell your friends or family to do the same and if they keep making new accounts and adding and trying to talk to you or your friends or family then you can block and report and you can also make your account private so no one can add you or message you