When the barber is saying:
"Do you shave yourself? If not, come in and I'll shave you! I shave anyone who does not shave himself, and noone else"
it means that he is a barber who obviously works in his shop during working hours, i.e. from 09:00 until 17:00. By the law, this must be clearly visible at the entrance door of his barbershop.
Before that time or after that time he is not working as a barber and he can easily shave himself; obviously he shaves himself.
The time element must be added to the equation.

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