Both things are protecting from different things. Since you asked about installing things twice, you did not understand how this sandbox method works. I already told all there is to it. So maybe read other explanations on the net on the topic, maybe they explain things better.
Just be aware that itch is not steam. If you only play popular games, sandboxing should not be necessary. And by popular I do not talk about that shiny new game you saw on steam that is now free on itch - those are malware most of the time.
I was only told to maybe not use the newest games and see if someone mentioned a lot of problems viruses or odd things in the comment section. I also usually stay away from stuff that doesn't even show any pictures since that gives me a low effort feeling.
Good advice. But be aware that it is trivial to copy description and screenshots from an existing project and publish it on an old hacked account.