The same value is repeat when we check more than one chkbox

Question

When i check more than one check box it repeat the same value like below

 

when i clik 1 means it seems 1

 

when i click 1 and 2 means it seems 1,1,2 

 

 

 

string val = string.Empty;

StringBuilder sb = new StringBuilder();

foreach (ListItem it in cckk.Items)

{

if (it.Selected)

{

val += sb.Append(it.Value + ',');

ViewState["vallue"] = val.Remove(val.Length - 1).ToString();

}

}

string a1a = Convert.ToString(ViewState["vallue"]);

}