My O2K has 2 Contacts folders because I want it that way, but if you don't want it that way in yours, then open the one you want to lose, select all the contacts and use "Move to Folder" to move them to the one you want to keep, and then delete the surplus contacts folder.
In "Tools - Services - Outlook Address Book - Properties" make sure that it is gone from there, otherwise use "Remove Address Book"