I've had to do this in a past job.
You have Excel, do you have MS Access? This would be fairly easy to do in Access. 2 features that you could use is find/delete duplicates, and append.
if you have a unique identifyer for each person (Customer ID, SSN, etc.)...then it'd be pretty simple, but if all you have is last name, first Mi, and street address, etc. then you'll have a few potential issues to think about when setting up your criteria for the search/delete:
same name, but different addresses (people moving, or different people)
same address, but different name (change of marital status, or different people)
because I don't know the content of your list...I'm not sure if these 2 circumstances would apply.
hope this helps a bit