I have the following table (Excel 2007)

Month Broker #
December 32350
December 32350
December 24715
January 24715
January 24715
January 44063

(table is a1:b7)
I want to search for a month and then count the unique records in column B.
So for the month of January, it would return 2 since one of the 3 records is
a duplicate. Any ideas?