change your LINQ statement as below
var classname = sms.createClasses.GroupBy(i=>i.ClassId)
.Select(g=>new{Classname= g.First().Classname, ClassId =g.Key})
.ToList();
rddlclass.DisplayMember= "Classname";
rddlclass.ValueMember = "ClassId";
rddlclass.DataSource = classname;