Yes, always backup! 

It is best practice to always have a recent backup of your valuable files before starting any disk cleanup project with Duplicate Cleaner; preferably on a separate drive, or in the cloud.  Large capacity USB drives have come down in price a lot in recent times - you'll get a lot of convenient backup space for your money.  Cloud storage providers are also a popular way of creating secure backups.

Note that the System Restore feature built in to Windows does NOT back up your personal files and documents - it is just for internal system and program settings so should not be relied upon to recover documents.