Just as an extension of the existing answer(s):
When visiting AU on a daily base, it is easy to forget the "language" and rules here are not the average, and the perspective of an "answerer" is quite different from when you have an incidental problem. A misconception on what makes sense is therefore easily made by new users.
What would you do if you were a newby on an OS you were not familiar with? Used to Facebook- like sites, Googling, you run into a site that seems to answer questions. Would you read the whole introduction on how to behave? I wouldn't, and actually I didn't.
The main reason why I didn't post duplicate questions in the beginning was the fact that I found it easier finding an answer than taking the effort to subscribe and ask. The whole concept of (avoiding) duplicates, the voting system, privileges (many times people think editors are moderators...) it all starts making sense over time, coming from the average Internet habbit.
That doesn't mean posting duplicates (or neglecting the voting/accepting system, posting "thank you" as an answer etc, etc) should be accepted, but
...sigh... you do WRONG [downvote, downvote]
Is rarely the shortes way to achieve what you want: people to understand what is the "accepted" procedure here.