dlvr.it will not post an item unless we find it in your feed at the time we retrieve the feed and make the post.
There are a few known issues with blog systems and third party feed hosting services that can cause dlvr.it to post old items from your blog/feed:
Some blogging systems can 'hiccup' and re-post old items in your feed. This most typically happens when the user makes significant changes to their blog configuration / settings. In this case, old items are often given new GUIDs and are seen by dlvr.it as new items. However, there are cases where the blogging system has spontaneously re-posted old items to the feed.
Some third party feed providers (most notoriously Delicious) spontaneously hiccup and repost old items to the feed. This typically is a brief hiccup and by the time you check the feed, the old items have dropped off the end of the feed and only the newest items are then present in the feed.
If you remove items from your blog, it is possible that these items are also removed from your feed. This allows older item to bubble up into your feed. If dlvr.it hasn't seen these old items before, then dlvr.it may post them to your social networks.
If you edit an old item from your blog, some blogging system will move that revised old item into your feed. This can cause dlvr.it to unexpectedly post an old item.