The Remove Duplicates feature only removes duplicates of the site:user:password combination. I'm not sure what the original poster meant, but it would be nice to be able to get a list of sites that are using the same password across sites. Right now LastPass gives a warning that a password is duplicated but I'm not sure if there's an easy way to identify what it is a duplicate of without dumping the full password list into a text file and sorting it in Excel or something.
This feature was in the old "Security Challenge". It is still accessible on mobile (at least on Android, can't confirm for iOS) under security -> security challenge. Then click show me my detailed results. I usually have that open next to my computer when I do the cleanup.
Thanks for the feedback. I've passed this idea along to our development team for consideration in adding to future updates to the Security Score area of the Security Dashboard. You can still view the Detailed Results as described above on iOS as well as Android, to view these details on the iOS app you would tap Security > Security Challenge > Start the Challenge > Details in the top right corner.