Why do some people get bent-out-of-shape about duplicates?

That depends. I will split groups of similar species that tend to grow together, highly variable species or poorly observed species into single individuals/ observations. I think this makes sense. However, I got annoyed in the past by an (otherwise great) observer who split schools of fish into separate specimen. I was annoyed by scrolling through hundreds of nearly identical looking fish, even told him off for that. He told me he is interested in individual patterns of this species, which makes his modus operandi understandable. Still, somewhat annoying …