Research how Tumblr and Twitter handle comments/reblogs for previously seen posts
When a Briar blog post receives a comment or reblog from someone whose blog we subscribe to, a duplicate of the post is added to our combined feed. Research how Tumblr and Twitter handle this, specifically in the case where the comment chain forks (for example, Alice comments on the original post and then Bob comments on the original post rather than replying to Alice's comment).