Comparing data in two different spreadsheets

i have two databases, each in which has generated a list of customer names, phone numbers, etc. These lists are both in seperate workbooks in excel format. Each also has over 15000 entries. I would like to somehow merge this information and pull out any name that is not found on both databases, I also want to pull out any name found on both docs more then once. My goal here, to give you insight, is to delete any customer account that isnt provisioned on both databases, and also to purge some of the many duplicate accounts that were provisioned on both, but more then once. PLEASEEE help- I know this can be done, as I have been provided this information before, Im not sure how to do it. Please email any information you think would help. Thanks so much