Urgently ....How to avoid duplicate items being added from one CheckBoxList to another CheckBoxList ?

I  got 4checkboxlists , each checkbox has 6 checkboxes inside it (below is my database)
checkboxlist1 : rowa
checkboxlist2:rowb
checkboxlist3:rowc
checkboxlist4:rowd


id : int rowa : bit rowb : bit rowc : bit rowd : bit rowtext:nvarchar(50) My problem is , When adding items from one CheckBoxList  to another CheckBoxList
 ,how to check if the item is already present in the second  , third , forth  CheckBoxList ?

this is mycode 
Protect sub_button1 'A CheckBoxList For a As Integer = 0 To CheckBoxList1.Items.Count - 1 If CheckBoxList1.Items(a).Selected Then rowtext = CheckBoxList1.Items(a).Text rowa = 1 rowb = 0 rowc= 0 rowd= 0
...