Most of the time, Organizers will tell you not to keep multiples of the same thing. Why would anyone need more than one copy of a book? Simplify! Edit those t-shirts/ sheets/ mixing spoons/ board games!
But in the case of your computer, and electronic files this is a yes-and-no situation.
Yes, you should sometimes have a clean-out, and delete documents, bookmarks, and programs that you no longer use.
But we have all been warned about backing up files and programs that we do use. We do need duplicates of our digital photos, documents, contacts, and anything else we count on. I have been told by people much more knowledgeable than me, that it’s not safe unless you have it in at least two other places. That’s a lot of backup.
Remember floppy disks? That was a good backup system, until your new computer couldn’t access them. When you use USBs, CDs, DVDs, or any other format, make sure they are up-to-date.
For your second backup, unless you have corporate or national secrets, I am going to recommend off-site “cloud-based” storage. My current favorite for documents, files and pictures is Dropbox.
If I can figure out how to use it, anyone can. It’s super-easy to save information to your account. It will let you store a fair amount of stuff for free. You can share a file with someone else, if you need to collaborate. And you can access it from any computer, at home, work, or anywhere.