Find unique values in column - in a SINGLE CELL FORMULA; can it be done please?

Hi all,

On the attached sheet I have a column of emails (in my real version, this column could contain up to 500k or more emails).

I know how to count the number of emails in the column BUT many are doubled.

I want to add a SINGLE CELL FORMULA in cell D2 which will count the number of unique email values in the entire column A.

I don't want to add a filter in column B and count that.

Can this be done please? Played with several SUMPRODUCT ideas via Google but most 'freeze' up due to process weight.

Thanks in advance,

Ian