CCleaner found a number of duplicate files on my C drive. Is it safe to delete all but on of the files when there are two or more duplicates?
No, bulk cleaning would be bad, many operating system and program-related files are amongst that list. Take your time, go through it, only remove files you know you don't need (documents, mp3s, videos, etc.)
And it wouldnt hurt to have a personal backup of those folders prior to removing files, and even a system image would be recommended - just in case.