how to solve this erro : "A column named 'Rowno' already belongs to this DataTable"

hi i am merging value in datatable inside  foreach.  foreach works using id iteration. id = 1,2,3,4
foreach (ListItem list in chek.Items) {
qlConnection pubconncheck = new SqlConnection(connstr);
SqlCommand cmd4 = new SqlCommand("select * from table where id = id_value); SqlDataAdapter sp_adap = new SqlDataAdapter(cmd4);
DataTable chktb1 = new DataTable(); DataTable sp_dt = new DataTable(); sp_adap.Fill(sp_dt); DataColumn column = new DataColumn("Rowno"); column.DataType = System.Type.GetType("System.Int32"); column.AutoIncrement = true; column.AutoIncrementSeed = 1; column.AutoIncrementStep = 1;

chktb1.Columns.Add(column); chktb1.Merge(sp_dt);
}

here first iteration works correctly. 
when it goes to second iteration i am getting this error : "A column named 'Rowno' a ...