It depends on what you mean by “saving”. If you mean accepting the shared album request, it’s possible that this isn’t duplicating the storage, though I’m not 100% sure how that works (it may only “exist” once in iCloud, but it’s still taking up room when it’s viewed from the phone anyway, since the device needs to at least temporarily store the photo to show it).

If you’re talking about saving a photo from a shared album into your own library, that is without a doubt not referencing the original and is now a separate file in your own photo library