Merge two ABBU address book files into one

I have one Address Book ABBU file from a macbook that I want to merge with the current abbu file on my mac. I have about 100 more contacts in this old file that I would like to add to the current address book that I was finally able to sync between my iPhone, iMac, MacBook and Mobile me. When I try to "import" i get the are you sure you want to overwrite question/error message and I do not want to overwrite. I want to merge the older file with the active and current address book and delete duplicates (I think that is what i want to do?)
Thank you