Hi,
welcome to So Many Me. The "Yes, I' m ME too!" achievement will be the first achievement you ll get in the game and it is unmissable.
In world 1-1 just follow the path untill you will trigger a cutscene with an egg. After the cutscene you will have found your first other ME and the achievement will unlock.
Just in case you don't know what I am talking about, jump to 3:05 in the video below. The green thing is the egg I am talking about.
(video credit: emozaga)
Hope this helps you guys!
Any improvements? Please let me know in the comments below!