How to use duplicate finder. Should one copy of each file be left unchecked? If so, this would be a very tedious process.
It is deliberately meant to be like that, so that you have to think about what you are doing and don't delete the wrong things.
Some files, especially some system files (.DLL's and the like) NEED to have duplicates in different locations for your computer to work properly.
If you could just delete them without thinking about it then you could very easily break your computer.
Unless you are very experienced then you should only use the Duplicate Finder to look for duplicates of things that you have put there - Photos, Videos, Music, Documents, etc.
Leave any/all system and program files well alone.
And then it's still up to you to decide which you want to remove (if any) and which you want to keep.
Nobody else can decide that for you, espically not a program.