Just thought I would followup on this one in case anyone else has the same issue.

I'm pretty sure this can't be done with an expression without custom code, maybe that will get me a response 🙂

What i did to fix it is to alter the Stored Procedure that was providing the data. I only include a value for gender on what i have called the 'master row' for an individual person, otherwise it's left null. Then a count of the value is accurate because the duplicate rows do not also contain the gender. I knew all along that this was one way to do it, but i wanted to do it in SSRS!