To avoid clasihing and give thepop up message

design as follows in Data Grid VIew;
 

Date Session RK VK SK MK CKK CMK UNNI MS TMS
 

7/1/2013 1 REO PH2 REO
7/1/2013 2
7/1/2013 3
7/1/2013 4

 
In the run time 7th date lst session REO (course) is taken by RK (Staff).then that same course REO is not to be taken by SK (Staff).
 
suppose SK select the REO course in the data grid view message would come as"RK taken REO Class would you accept again" the message should be the yes or no.
 
if yes SK allowed to take REO on the 1st Session.if no SK not allowed to take class.
 

for that how to do validation for clashing using c sharp in Data Grid View.