Isn't hei hei = hi hi ?
Nope. When it's said only once, it's a greeting. When it's repeated, it's a farewell.
what about moi?
why is "bye" only not accepted?
Report it. It should be.
I thought moi was bye
Moi is both hi and bye, more often used as hi, same for hei
and moi moi or hei hei is bye only