Count Months that occur more than once in the data set but ignore Months that show once

Hi,
i am wondering how to count months that occur more than once in the data set below but at the same time i need to ignore months that only occur once (e.g May and July). The formula i used =SUMPRODUCT(($C$3:$C$21<>"")/COUNTIF($C$3:$C$21,$C$3:$C$21&"")) returns number of months as 7 but it actually the correct result is 4. If you could help me with this that would be great! Thank you.
Personnel Number Date Month Days
Chris W 5/10/2013 May Fri
Chris W 6/6/2013 Jun Thu
Chris W 6/13/2013 Jun Thu
Chris W 6/14/2013 Jun Fri
Chris W 6/27/2013 Jun Thu
Chris W 6/28/2013 Jun Fri
Chris W 7/12/2013 Jul Fri
Chris W 8/16/2013 Aug Fri
Chris W 8/19/2013 Aug Mon
Chris W 8/20/2013 Aug Tue
Chris W 8/30/2013 Aug Fri
Chris W 9/6/2013 Sep Fri
Chris W 9/20/2013 Sep Fri
Chris W 9/23/2013 Sep Mon
Chris W 9/27/2013 Sep Fri
Chris W 9/30/2013 Sep Mon
Chris W 10/25/2013 Oct Fri
Chris W 10/28/2013 Oct Mon
Chris W 12/13/2013 Dec Fri