Crystal Reports Build and Display Unique Values

Message

I need to build multiple dates in one field that is grouped by a customerID.  This process is working with the exception of it repeating the same date more than once.  There can be multiple items for the customer on the same date but I just want the UNIQUE date to be displayed once and not for each record line.  Can someone guide me as to where I am going wrong and if there is a solution for this?  Greatly appreciated. 
For instance I get 12/23/2013, 12/23/2013, 12/28/2013 if the customer had more than one item on the same day.  I would like this to display as 12/23/2013, 12/28/2013.

This what I am using to build the date s in the field:
whileprintingrecords;
stringvar CertainDates;
if CertainDates<> "" 
then CertainDates:= CertainDates+ ", ";
CertainDates:= if {Command.FieldName} = 1 then CertainDates+ ToText (({Command.Date}),"M/dd/yyyy " );
Then I am displaying them in the CustomerID group footer:
whileprintingrecords;
stringvar CertainDates;
CertainDates;

This is reset on the CustomerID group header:
whileprintingrecords;
stringvar CertainDates;
CertainDates:= " ";