THAT'S EXACTLY WHAT CLONES ARE!They are either alt costumes with their own personalities or ungodly servants of Great Old Ones.
Only time will tell.
They share the same attacks, weight, speed, that stuff. They have slightly tweaked differences, like Dark Pit has more horizontal knockback, Lucina has no tipper, Daisy's butt bash sends you down instead of the opposite direction you're facing (that's more from an article's statement though). Echoes are nothing more than full on clones.