What to do when having problem with power button of Samsung Galaxy S Duos 3?

for unlocking I am pressing power key at that it won't work, By pressing power key on while the phone switching off and switched on automatically. There is any suggestions to solve this without any replacement . If suppose I go to service Centre how much it will cost

I suggest visiting a service center. I think you broke the power key. You can contact Samsung to know total cost of repair.

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