If not, why not? I guess it would make sense to reject guest blog post offers if they are offering to write the same kind of stuff you already have. But what if the blog post offer actually enriches the value of your existing blog post inventory by adding humor or a quirky point of view? Still no deal?
Guest blog posts work on the principle of WIN WIN
You win by getting content that your users would enjoy or would stand out from auto-fed morphed stuff you're stocking your blog with. They win by getting a backlink.