Finding unique values for a date range

I need to find unique users from a particular location that appear between two dates.

I can find all entries (in column P of the My Marketing sheet) within the date range (defined by E53 and F53) and within a particular country with:
=IFERROR(COUNTIFS('My Marketing'!P:P,">="&E53,'My Marketing'!P:P,"<"&F53,'My Marketing'!E:E,"Australia"),0)

And I can find all unique entries in the column D with:
=SUMPRODUCT(('My Marketing'!D:D<>"")/COUNTIF('My Marketing'!D:D,'My Marketing'!D:D&""))

But no amount of experimenting or googling has lead me to the solution that allows me to join these together.

How do I adjust the first function to limit the results to unique ones?

Thanks for you help.
Gav