Often times people just starting to learn or are new to a subject matter dont know what they dont know. So they might simply not have the capacity to realize they are asking a similar question or that an answer to another question will suit their needs. I feel its more beneficial to allow these people to ask their questions with the limited expanse of knowledge they have and use it to learn. Hopefully as a result will one day return the favor. The system now shuns these people and they never return or help and end up disliking the community.

>By encouraging post duplication, you will quickly burn out folks who want to be helpful

This is what the voting system is for. Any system that places a ranking level on votes should filter this out easily. There is nothing that says you have to answer a question just because you know the answer. Just vote and move on to keep yourself from burning out.