Windows 10 twice on the same machine

You can use multiple copies of Windows 10 in what's known as a Multi-Boot configuration. Often you'll hear of this as "Dual-booting", because most commonly only two different OSs are in question.

How To Geek has a nice tutorial about setting up dual boot with multiple copies of Windows, and a good explanation of dual-booting in general.

Legally, you need a licence for EACH Windows install you make. So if you want to install Windows 10 twice, you'll need to own two licenses for it, even if they're only running one at a time, on the same computer.

Similar has been discussed previously here on on SU:

• Install Windows 7 twice (dual boot) - licensing issues?