Should I back up my files first?

Yes, always back up!

It is best practice to always have a recent backup of your valuable files before starting any disc clean-up 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 features such as Windows System Restore do NOT back up your personal files and documents - they are just for system and program settings, so should not be relied upon to recover documents. On macOS, Time Machine does back up your files, if it is set up.

Deleting to the Recycle Bin (Trash on macOS) gives you a chance to get files back, but it is not a backup - files on some drives can’t go to the Recycle Bin, and it is emptied eventually. See Delete files.