Sometimes Sitecore 9 SIF installation script fails as multiple certificates found with same DNS name.
It may happened when you try to install Sitecore 9 using SIF and somehow it got failed due to any reason. You may tried to reinstall it. And If you tried to reinstall it with different user account, your SIF script might get failed with below exception.
Cannot process argument transformation on parameter 'Signer'. Cannot convert the "System.Object" value of type "System.Object" to type
I found the solution here and to continue with installation, I need to remove all already installed certificates but I don’t know, how to remove those from my system?