I need a formula/filter/something to help me out.

I have:

Column D: Last Name
Column E: First Name
Column F: Company
Column G: Address Line 1
Column H: Address Line 2
etc...

I know my list has many many duplicate records, and I need now how to get
rid of the duplicates and count those left. If a record shows up at least
once, I need it to count, but I don't want to count the numerous duplicates
in my spreadsheet.

Thanks for helping.