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What you are seeing is probably a browser hijack, most likely caused by a site you may have visited. You should be able to get rid of it by going into settings>apps. Scroll down to your browser and click on it. Click clear data.
I've received several of those free iPhones, or rather, would have if I'd clicked OK and entered my details, or rather, would have had my account compromised and phone filled with filth if I'd clicked OK and entered my details.