Board Thread:General Discussion/@comment-27064008-20160403041313/@comment-25348898-20160414124614

Welp, seems they're right, as Undyne's dummy mentions the Mad Dummy fight.

So there're only two Dummies in-game, but Nabstablook doesn't refer th Mad Dummy as "cousin" after interrupting the fight. Nabstablook acts as though (s)he doesn't even know them. So I don't think either of them are related to Nabstablook and Mettaton.