So you're seeing duplicate entries being posted to one of your Destinations...
Things to Check
Is there another service which is also posting to your account?
Services like TwitterFeed and NetworkedBlogs can be disabled either from the application's website, or by removing the application's access to your account via the "Settings - Application Settings" menu in Facebook.
Facebook Notes Import can be disabled by visiting the Import Settings page and removing the imports for the feeds that you are using in dlvr.it
Services can be disabled either from the application's website, or by removing the application's access to your account via the Twitter Connection Settings page
Does your feed have "Republish items with updated timestamps" turned on?
Edit your feed by clicking the pencil icon next to your feed's name
Select the Advanced tab
Uncheck the "Republish items with updated timestamps" checkbox.
Items will no longer be reposted when the content is modified
Have your item URLs changed?
A frequent cause of duplicate posts is that the item URLs in the feed have changed. One way that you can detect this is to:
Locate two duplicate posts and open the links in two separate browser windows or tabs
Compare the URLs displayed for each link in your browser
Pay attention to differences in the site name (www vs. no-www), changes in the dates and times which may be present in the URL or URL parameters. Any change in the URL likely indicates that the URL in the feed changed and that the item was detected as new due to that.
You can help prevent this from happening by ensuring that your feed has proper GUID tags, and by disabling any features in your feed publication software which may be causing the URLs to change.
Are your item GUIDs changing?
Along with URLs this is one of the most frequent causes of duplicate items.
A GUID is a unique identifier that represents an item. Sometimes the GUID is also the URL, because this does uniquely identify the item. If the GUID changes, we search our system for the new GUID, and when it's not found we are effectively being told that this is a new item which we have never seen before. Since the item is "new", we post it using the rules as if it was a new item. More information about GUIDs can be located in this support post.