We use a off the shelf point of sales program. Because of different
spellings of street names, spouses and other people in the household there is
a lot of duplicates and errors. When we want to do a mass mailing we get the
data exported into an excel file.

I have a cleaned up version of the master file with a lot of the dups
eliminated. There is a data field in the worksheet from the vender that has
edit date. I want to just take all dates from the last import and import
that information to my mailing list file. This would include new records and
updates on existing records. There is an existing unique primary key field.

So if the primary key field is already present in my cleaned version I want
to overwrite that with the new data. If it is a new primary key number I
want to append onto the bottom of the file.