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Back to your question - This has nothing to do with Lightroom. If an image is imported into Lightroom the file is not in Lightroom. You need to simply backup your hard drive, which contains your image files as well as the Lightroom catalog containing information about your images.
I have the copy on my HD, a copy on a "backup" drive plugged into the computer and a second backup that is stored off-site in a safe deposit box. I used to keep it in a fireproof safe in the cellar, which is not too bad.
In my case the safe copy is the only true backup copy. I swap the backup on the computer with the safe copy every 1-2 months. The frequency is dependent on how much you are wiling to loose in the case of a fire, flood, theft, lightning strike, etc, etc. As others have said, cloud is only practical up to a certain size. I have used it in the past but now have about 400 GB and it no longer makes sense. Also, I would not trust it exclusively since I do not have complete control.