As far as duplicates are concerned, I believe that if you look at the context of the game (rather than the developers laziness/or whatever), some spells are actually renamed for a reason. Yes they have everything exactly the same (stats wise), but perhaps the spellcaster who created the spell in Canthas has a different approach to casting it than the ones in Tyria. And thus has a different name.
Now, as why they have some as "core" skills and other that are deemed "new" I'm not sure. Some are definitely new, but others, as we have pointed out, are renamed old skills. Why the special treatement of certain old skills? Factions only players should have these skills. Prophecies have their own. The ones who bought both, can now have better uses for those skills. A Necro using touch skills? Now that you have 2 Vampiric draining skills (Touch and Bite), perhaps its now more beneficial to use touching attacks. (To quote Nintendo: "Touching is good").
Some skills that maybe used less than others, are now more encouraged to be used, as now you can use them more often than before. The more useful skills that "everyone" uses, don't need to be duplicated. There is use, and there is abuse (casting certain extremely useful skills that have a long recharge time - twice).
