Column D = text values (client names) where the same name could appear more
than once in the column and where there could also be blank rows.

Column H = contains "Success" or "Failure".

I want to count the number of UNIQUE clients where I have had "Success" as
an outcome, as in the example below, where the answer would be 2:

Column D Column H
AlphaX Success
AlphaX Success
DeltaD Failure
CharlieW Sucess