So, let's say you have an older entry on your blog that is from a few years back, could you just republish it as new or would that cause a duplicate content issue and possibly affect your rank in Google?
I don't think that Republishing an older content within the same blog will effect the blog's ranking or cause duplicate issue. As the content is within the blog and the blogger can republish his news again for his readers who have missed that entry before.
Every time you hit the publish button on Wordpress it resubmits the article to search engines and then pings the search engine to re index it. Therefore it is best to only edit when you need to and do it as few times as possible. However, editing old articles to add new content shouldn't hurt.