Participation in the mailing lists, like participation in any community, requires a common basis for communication. Many of the mailing lists presuppose a knowledge of the Project's history. In particular, there are certain topics that seem to regularly occur to newcomers to the community. It is the responsibility of each poster to ensure that their postings do not fall into one of these categories. By doing so, you will help the mailing lists to stay on-topic, and probably save yourself being flamed in the process.
The best method to avoid this is to familiarize yourself with the mailing list archives, to help yourself understand the background of what has gone before. In this, the mailing list search interface is invaluable. (If that method does not yield useful results, please supplement it with a search with your favorite major search engine).
By familiarizing yourself with the archives, not only will you learn what topics have been discussed before, but also how discussion tends to proceed on that list, who the participants are, and who the target audience is. These are always good things to know before you post to any mailing list, not just a FreeBSD mailing list.
There is no doubt that the archives are quite extensive, and some questions recur more often than others, sometimes as followups where the subject line no longer accurately reflects the new content. Nevertheless, the burden is on you, the poster, to do your homework to help avoid these recurring topics, and especially the dreaded bikesheds.