If You are Installing windows for the first time and somehow you have installed the OS twice on your system. In this situation you can see the choice of OS or option to choose OS from dual boot menu like when you start your PC then It Shows the Brand name of your Hardware system and then it goes to your OS loading windows there you can see dual boot menu i.e. if you have installed windows 10 twice then you can see two OS with the same name windows 10 there you have to choose the OS you want to boot.
In this situation you have two situation you can reinstall OS or you can just delete the one installation from the other drive than c which is accidentally installed by you.
Option 1 :-
Now If you would like to reinstall windows OS, which means removing all your apps and personal settings and files, you can use reset or refresh. This process is equal to a clean install.
If you have created a installation DVD or Bootable drive, you can use that to install windows. First, you will need to reinstall the qualifying license, then start the upgrade gain to windows.
A workaround for this situation is to create a system image of Windows just after you have upgraded. So if your drive does crash, you can simply restore the image on a new hard disk or restore the image on the same drive if you just want to reinstall Windows .
In this another option I will tell you some other this you must know about your installation if you have installed same OS on same drive.
When you install same OS on same drive then when you install OS second time the all the files form OS installed at first time is saved to the folder called WINDOWS.OLD . Just find that folder in your C drive and delete that folder.