Lexaraj

How does breeding IVs, and IVs in general, work in this? I know it wasn't possible to reliably breed perfect IV Pokémon in Gen 4 but it was relatively easy in Gen 6 and 7. We're there any changes made to breeding on order to do this? Are there any other ways to increase/alter IVs?

By 'duping' I mean by just setting the same copy of a Pokemon in empty slots. The built in legality checker shows they're legal but I didn't know if there was something more specific that I needed to worry about. Anytime I duplicate a Pokemon or change it's species in PKHeX, I always reroll the PID. Am I good to go doing all of this, or do I need to be doing something different?