I have a sorted column of account numbers that I want to count.

I don't want to count more than one occurence of an account number if it has
the same work date as the preceding occurence. But differing account numbers
with the same work date should be counted.

Account # Work Date
====== =======
A6666666 09/10/08
A6666666 09/12/08
A5555555 09/12/08
A4444444 09/11/08
A4444444 09/11/08
A4444444 09/12/08
A4444444 09/13/08

In the above example I want to count both occurences of "66666666' because
they have different work dates. I want to count only three occurences of
"44444444", the first, third and fourth. My count total for the above
example should be "6".

How can I do this?