Shoutbox

Your contact list is not stored on the hard drive, but on servers. I don't have an solution for your actual problem, but I would simply try with uninstalling WLM and install it again, but I don't think this will make a difference, as the contacts are, like I said, on a server so if you indeed made duplicate entries of each contact they would not be removed from your contact list.
However, the protocol WLM uses should, AFAIK, prevent duplicate entries (two contacts with the same e-mail), so it sounds a bit odd that you have identical copies of your contacts. When a contact comes online, does he or she only come online on one of the contact entries?
If you feel uncomfortable with removing WLM entirely, you can backup your contact list into a file by choosing Contacts on the main WLM window and then choose Backup contacts (I'm not able to give you the exact English name, but you should be able to figure it out - it's at the bottom of the Contacts menu). Note that the file is no more than a text file with a different extension, which contains a list of e-mail addresses. WLM will continue to retrieve contact information from it's servers even after the backup.