Duty-free import only applies if you are bringing something into the country that you bought outside it. If you take a laptop out of the country and bring it back, that isn't import and you don't have to pay duty.
Now the trouble is, you have to convince customs officials that you did in fact take it out of the country. Much of the time they may take your word for it, or not even ask. I've never had any trouble (though I've never travelled through Indian customs). If you have the receipt, or a guarantee document indicating where it was bought, that should work. Alternatively there may be a way to register your laptop on the way out of the country with customs, or at least record the fact that it was taken out. You will be less likely to have trouble if the laptop doesn't look brand new, and has personal stuff on it.