I recently got a football columnist to cross-post on my site, meaning he is posting on my site and Blogcritics. Will this make Google devalue my site? Will it apply to the entire site or just the post pages of the duplicate content?
Your pages may end up in the supplemental index of Google, especially since your site would probably be crawled after the newspaper sites or where ever the columnist writes. You probably won't suffer huge penalties, but the benefit of having the columnist write, in part, on your site may be great for your readers, thereby negating the penalties for duplicate content.
Google's dupe content filters are largely a joke. Rarely, they are an issue. In these cases, it's generally true that whoever is indexed first takes the credit. Sometimes, if one site VASTLY outranks the other, and stays pretty tight to the niche, it can break the first indexed rule. If they backlink to your article though, you should be safe.
Very true. Especially this part: "In these cases, it's generally true that whoever is indexed first takes the credit."