The "unlimited" data you're talking about is for data access with your phone, not for use of your phone as a hotspot for another device. So, to use your phone as a hotspot, you need to pay extra for a special data allowance for hotspot usage. You can find the hotspot add-on optiions on Boost Mobile's website.
You're not paying for the same data twice. The plan for your phone comes with unlimited data but that is for your phone ONLY. If you want to tether, you need to add the hotspot to your phone which will then allow you to not only tether your laptop but any other device as well like an iPad or any other tablet.