Unique Bookmark was done Team Webgali for me..  The Main Idea of this plugin is to not allow users to bookmark the same URL multiple times. Working method:

1) The plugin does not allow an user to post the same URL again instead it gives a MSG stating this URL is already bookmarked.

2) It neither allows an another user to bookmark the same URL (Say if Jeff is bookmarking an URL & John tries to bookmark the same URL it does not allow John instead it asks John to rate / share the bookmark)

3) Say if John post a URL in private and the same URL is posted by Jeff in public it allows him to post. The very main aim of this plugin is to reduce the spammer who keep posting same URL again & again.. Bugs that i found:

1) Once an URL is posted by the owner it does not allow the same owner to even Edit it (It's actually a bug that needs to be fixed)

Don't know Stuffs: 1) I don't know how it will work in walled garden.

I am not a PHP coder if anyone is interested in developing this plugin further your most welcome but plz do share your development with the community.. https://github.com/merilturock/unique_bookmark