1) Look at how feats are written. They are written
Prerequisites: ...
Benefit: ...
The duelist class ability says you gain the benefit. It says nothing about the prerequisites. You get what it says (the benefits) and don't need the rest (the prerequisites). It even goes further to modify the benefit by saying you don't need a free hand to use it--you'll find that most "free feats as class abilities" work this way. So specifically, you do not need IUS or 13 dex to use Deflect Arrows as granted by Duelist.
2) Yes, you can still take it as a feat assuming you meet the prereqs--there is nothing that precludes you from taking it. Depending on whether you can use it more than once per round by having it from two separate sources is an issue your GM needs to decide as it's the same benefit and there are conflicting opinions about the same benefit from separate sources. Most GMs will tell you no as these two specific benefits even come from an ability/feat with the same name.
3) It's not explicitly written in the book one way or another. The answer should be yes, but that will be up to your individual GM. We can fall back on SKR's famous post (famous to pathfinder rules forum nerds..) with a quote from a forum post answering exactly what you're asking.
If it walks like a duck and quacks like a duck, it's probably a duck. If you line up Daffy Duck, Donald Duck, Duckman, and Howard the Duck, from a game standpoint it makes sense that a +1 duck-bane arrow is going to do +2d6 damage in addition to normal arrow damage if you shoot any of them, because they're all ducks. And if you shot that arrow at "Duckie" from Pretty in Pink, it wouldn't get any bonus damage, because he isn't a duck. And you should be able to see why those first four are ducks and the last one isn't.
source for quote http://paizo.com/threads/rzs2ln8z?Can-a-Life-Oracle-with-Channeling-take#30
