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It is generally OK to hire and I agree with Samer's assertions below.
I would also add that if the fear of hiring is based on the over-qualified candidate leaving quickly you can require an employment contract or other mechanisms motivating them to stay for a specific period of time.
its ok and needed even to hire an over qualified candidate for a certain position in the company when your company is not performing well at at a certain deprtement say but not limited for example at the markeing departmement or the uper management however such a person who is over qualified must be very well payed in terms of salary and allowances