Are the new records going to replace the equivalent records in the old
sheet? If so, then you can add this formula in a blank column of the
old sheet:

=IF(ISNA(VLOOKUP(A1,new!$A$1:$A$5000,1,0)),"unique ","duplicate")

and copy it down to the bottom of the 15,000 records. This assumes that
Application_Number is column A in both sheets, and that the more
up-to-date sheet is named "new" - adjust these to suit. The formula
identifes those records in the old sheet for which there is a duplicate
in the new sheet.

I would suggest then that you sort the data in the old sheet in
descending order using this new column, so that all the "duplicate"
records fall to the bottom. These can then be deleted, and all the
records from the new sheet can then be pasted in at the bottom of the
unique records. You can delete this helper column, and re-sort the data
on the key field.

Hope this helps.

Pete