In a post I wrote earlier this month about pre-requisites to knowledge sharing, Ellen asked about what do when the same old question keeps popping up even if it has been resolved or hashed over ad nauseam in the past.
This question highlights what the title of this post sums up: there is a difference between rehashing the invention of the wheel vs. learning about how to use it.
Many knowledge sharing/capture/management/what-have-you efforts are developed specifically to avoid reinventing the wheel. What is often overlooked is that you then have to educate new members to the effort about what has been covered in the past. Often, the only way to do this is when the question arises. Rather than getting in a twist about someone asking about wheels, view it as an opportunity to connect them with what has already been discussed/developed about wheels.
The Strobist blog gives a good example of how to deal with this. This blog has coalesced a community of flash photography aficionados who interact via the blog comments and their Flickr group. Many newbies join this group every day. David Hobby, who writes the blog, has written a document called Lighting 101 that covers the basics of off-camera flash lighting. New participants are gently and quickly directed to it so they can then interact with some knowledge in the broader community. Same thing can be done in other environments as well.
Methods for guiding people to these foundational resources are quite varied and include everything from very prominent links, tailored local search engine results and members of the community acting as knowledge sherpas.
Even when your wheel has been invented, there is still a big need for others to learn about how to use it. Make sure that is addressed in your efforts.