At Admin Panel -> Settings -> Switches, the "Link checkers" switch enables utilites to check for dead, non-reciprocating and duplicate links as well as undesirable content. This is normally only applicable to WSN Links, not the other scripts, though you could use it anywhere you have the URLs switch on. The link checkers are located in the Admin Panel's "Links" menu.

The duplicate link checker is intended to allow large directories to avoid accidentally adding the same link two or more times. The duplicate link checker will find all instances where there are two links pointing to the same URL and mark them as suspect. If you have purposely put copies of the same link in multiple categories, do not run the duplicate link checker... it would suspect all your intentional duplicates. (Note that if you have aliased links, it will not suspect these.) When the check is complete, you can review the suspects and decide on appropriate action. Alternatively, you can check the automatically delete duplicates box at the beginning to auto-trust the checker. You also have the option to check for duplicate reciprocal URLs in case someone's trying to cheat you by using the same linkback for several listings.