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?
The duplication can hurt you in a couple of ways (as @closetnoc highlighted) so I'd take a different approach.
You can add alternate URLs for previous versions of the page, but keep the original URL as the primary/canonical. If your original URL is /page/name then you could use something like /page/name?v=2016-03-01 or /page/name/v1 to link to previous versions. Each of those pages could include a <meta name="robots" content="noindex"> tag to keep them from being indexed (or a canonical to the original URL if you're so inclined).
This approach allows you to regularly update your posts to provide fresh or more current content whilst still providing an archive of the page's history.
Google uses semantic scores for pages to find duplicate content. This is not a linear comparison, but an extensive matrix of semantic scoring that very easily and quickly indicates when content is a duplicate of another. Duplicate content does not have to be exactly or nearly duplicate anymore. It just has to be similar enough.
There are a few actions Google can take.
1] Simply chose the first document found as the original and only index that page.
2] In the case where enough duplicate content is found chose to determine that the site is engaging in spam and penalize the site in the SERPs.
Neither will do anything you want.
There are a few things you can do.
1] Update the original post and remove the newer posts.
2] Noindex the old posts.
3] Use the canonical tag to indicate which post Google should pay attention to.
There are advantages to each of these.
1] Updating the original post allows you to take advantage of existing links, PageRank, and past performance metrics for the page. As well, you will advantage any user who has bookmarked your page.
2] Noindex will remove the old posts from the search pool and target your new post, however, it will lose the advantages in point #1.
3] Using the canonical tag will indicate which page Google should pay attention to and prevent a duplicate content issue, however, it will lose the advantages in point #1.
Given this, the best thing you can do is to update the original post.
