The goal of the StackExchange Q&A sites is to become a complete resource for all information on any given subject, and that a Google search will lead to said site, which has an in-depth, carefully edited, correct and up-to-date answer.
The problem with blogs, forums and FAQ-type sites is that they tend to become outdated very quickly. They're also very biased, as they're written once by one person, and there is very rarely discussion and editing.
So maybe somebody goes around asking the same question to several sites, but hopefully the question and answer on StackExchange is the one that is kept alive and up to date, because other posts on other sites tend to die off.
I don't agree with excessive content seeding, but I think this form of question duplication is perfectly fine to achieve the goal of becoming a central repository.