Hi, Andru. I'm not 100% sure I agree that this is a problem with the community per se. It is a problem for each individual in the community, because it's difficult to get a definitive answer to a question when the people who could answer it definitively are so tired of answering it multiple times already that they ignore it, leaving less qualified people to provide less definitive answers instead. But if you consider the community as a whole as an ecosystem, redundancy is one of the most notable features of the ecosystems we strive to imitate, so why shouldn't our own ecosystem be redundant as well?
That said, there are off-the-shelf solutions to this problem, courtesy of the open-source software movement. The one I'm most familiar with is the Drupal Project module (
http://drupal.org/project/project) which makes it easy for (among many other things) threads of conversation to be flagged as duplicates of other threads. Once that is done, the duplicate conversation effectively ends and discussion continues on the original thread. I'm not recommending Project module for a permaculture discussion site -- it's too technical. But on the existing sites where we're conversing, we could establish a tradition of flagging conversations as duplicates, just by posting a URL in the comments.
Another approach would be to use a wiki -- ideally MediaWiki for cross-compatibility with Wikipedia, if not Wikipedia itself -- to archive what is definitively known about permaculture research to date and highlight the remaining questions that need to be researched (for example, with "stub" articles). It is my understanding that this sort of research coordination is the whole point of the various regional Permaculture Research Institutes, but in practice they seem to have their hands full with other projects. Again, the approach would be to steer conversation about questions that have already been answered but are posted elsewhere to a central archive, being sure to keep that archive open for new contributions.