SSRS 2008, unique filter in drop down list

Hi guys…

i m creating a filter in SSRS reports and here i found duplicates , so i want to unique values in filter drop down list.

plz reply if anyone can.

thanks–

Harry

i was try this one

Countdistinct(fileds!fieldname.value)

but it returned the count of values not removes the duplicates from drop down list.