There are staff statements about preferred practices for multiple-species observations here and about duplicates here. The guidance on the first has been added to the FAQ page; @tiwane indicated he was going to add one about duplicates, but I don’t see it on the page yet.
I agree that additionally putting these things on a page more oriented towards the needs of IDers rather than buried among general “about iNat” questions would probably help make them more prominent and findable.