You have to do it manually. Cross posting is one of the tools sploggers use to push stuff out to multiple blogs at one time, so wordpress doesn’t support that.
Also having duplicate content on multiple blogs will kill your search engine ranking. Search engines suspect that one or more of the blogs might be a spogger/scraper, so they discount the ranking of both or all.
Yes, a spam blogger. Typically they steal content from others and then post it to tens or hundreds of blogs and surround the stolen content with all sorts of ads in hopes that they will lure some unsuspecting web surfer into clicking on the ads so they make money.