Basically, grub tries to find all the installed OSes on your computer. If, let’s say, Win8 was installed on second hard drive (sdb1 on a screenshot refers to 1st partition on the second drive), then you’ve upgraded it to Win10, and Win10 placed it’s bootloader onto a first harddrive (sda1) – then grub would find two bootloaders and correctly (from it’s point of view) list them both.
If that was one-time glitch – then @refi64’s command shall help you. @amitgold’s one was a bit incorrect, you definitely should provide grub2-mkconfig with a filename, or else it’ll just print out the new config on the screen, not save it.
If it’s persistent – then it’s very easy to rectify, but will require manually editing config files. As this post haven’t been answered for 4 days, maybe you’ve already solved your issue. But if not and you still want to do it – ping me, I’ll try to provide you with simple instructions.