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.