I revise some of the old posts on my site on a regular basis. I could just rewrite them and move on, but I'd like readers be able to see the previous version. 

What I've been doing is just putting up a new post with revised content - but I'm a little worried that having largely duplicated content is going to give me Google ranking a hit - is there evidence that there is a penalty to Google ranking, and also, are there other good ways of letting audiences see versions of a post?