#1 - Isolate out address elements into
\taddress1,
\taddress2,
\tcity,
\tstate,
\tzip.

#2 - Run through a CASS certified address standardization
program that does zip+4 appends. Do not bother trying
to write this yourself. You MUST buy it or rent someone
else's. They have the Postal database with all the street
synonyms and aliases, route number->friendly name, list
of legal address ranges per street, etc. There is NO WAY
you can code a program to do this without the postal database.

I've used a variety of them depending on accuracy desired vs
cost and speed. If you like, give me your list and I'll run
it through.


#3 - Merge/Purge. You need to determine what to keep VS drop,
right? Start thinking about the different ways people write
their 1st name (Rob, Robert, Bob), etc. Then add in typos.
Account for titles, etc. Suffixes. Then start adding in
confidence factors - you want to error on the side of sending
more or less mail? Oh, don't forget the married vs maiden
name or the multi-family / mother-inlaw living together.
Who exactly IS Mrs John Smith? Do you drop her if you have
another female smith in the household? Oh, and do you
want individual, family (single last name), and household
(multiple last names). What about company?

The better MP programs are amazing. Poorer matchode based
ones will consider a lot of obvious (to the human eye) dupes
as unique people. My home brewed one is matchode based which
means more false uniques, but I can't offer my corp one to you.

Dunno about Ben's geocoding answer. Rooftop geocoding is usually
way too expensive for casual use, and centroid is way to wide
for deduping purposes.

When we geocode stuff, 1st step is to standardize.
They just did it for you under the covers.