No, you will not end up with a dual boot system. If you upgrade to 10, 10 is what you will have unless you downgrade back to windows 7 which can only be done during the first 30 days. After that you would need to do a complete clean install of the Win7 OS to go back to it. Before upgrading to 10 I'd be sure you have a windows 7 restoration partition or installation media like a disk, AND have your windows 7 product key documented somewhere in the even you NEED to reinstall 7, you will be able to.
If you don't ever revert back to 7, no worries, at least you'll have it if you decide to. And NO, you cannot upgrade to 10 and then use your windows 7 license on another machine, just in case you were going to ask.
If you don't know your current windows 7 product key, you can retrieve it using Magic Jellybean keyfinder:
https://www.magicaljellybean.com/keyfinder/