There's a perfectly reasonable explanation for this, and there's an actual economy surrounding this reason in our society today. That reason is: sometimes people just look alike.
Any similarities between characters due to the same actor being utilized can easily be rationalized in-universe by concluding that either A) they're related somehow or B) pure chance. After all, we have actual people being paid to look like celebrities in our own society, not to mention all of the other instances of awkward social interactions people have when they approach someone who looks like somesomeone they know, but it turns out to be a complete stranger.