I believe this would be used in the same context in which English speakers say, "What, are you nuts/crazy?" in response to someone's idea.
Just some thoughts on normal:
'normal' implies some form of societal values being applied (norms). 'typical' means how common something is. Often when we say someone or something is not normal what we really mean is it is not typical, and in some situations, saying someone is 'not normal' can be offensive.