These columns don't currently have unique values. error when i click on my collection page.

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C#

ASP.NET

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;
//using System.Data.Entity;
using System.Data.Common;
using BalSample;

public partial class admin_collections : System.Web.UI.Page
{
ClsInsert obj = new ClsInsert();
protected void Page_Load(object sender, EventArgs e)
{


currentdate_txt.Text = DateTime.Now.ToString("dd-MM-yyyy");
total_txt.Text = "0.00";
generat_reciptno();
if(!IsPostBack)
{
generat_reciptno();
bind_monthyear();
fee_type();
bind_collections();
}
}

protected void fee_type()
{
DataSet ds4 = new DataSet();
ds4 = obj.add_feestypes(5,0,"");
string fee_id = ds4.Tables[0].Rows[0]["fid"].ToString();
fee_list.DataSource = ds4;
fee_list.DataTextField = "fee_type";
fee_list.DataValueField = "fid";
fee_list.DataBind();
ViewState["f_id"] = Convert.ToInt32(fee_id.ToString());
}
protected void Onreg_change(object sender, EventArgs e)
{

int reg = Convert.ToInt32(reg_txt.Text);
DataSet ds3 = new DataSet();
ds3 = obj.register(2,"",DateTime.Now,"","",0,0,"","","","","",reg,0,0);
string name = ds3.Tables[0].Rows[0]["stud_name"].ToString();
string stud_id = ds3.Tables[0].Rows[0]["sid"].ToString();
string batch_id = ds3.Tables[0].Rows[0]["bid"].ToString();
int bid = Convert.ToInt32(batch_id);

DataSet ds5 = new DataSet();
ds5 = obj.add_batch(7,0,0,"",0,"","","",bid);
string amount = ds5.Tables[0].Rows[0]["fees_per_month"].ToString();


name_txt.Text = name;
ViewState["s_id"]=Convert.ToInt32(stud_id.ToString());
ViewState["b_id"] = Convert.ToInt32(batch_id.ToString());


//double amt = Convert.ToDouble(amount);
ViewState["amount"] = Convert.ToDouble(amount);

bind_collections();
}
protected void generat_reciptno()
{
DataSet ds = new DataSet();
ds=obj.fee_collections(1,DateTime.Now,0,0.0,0,0,"","",0,DateTime.Now,0);
string receipt;
if (ds.Tables[0].Rows.Count > 1)
{

receipt = ds.Tables[0].Rows[0]["receipt_no"].ToString();
}
else
{
receipt = "0";
}

int receipt_no = Convert.ToInt32(receipt);
receipt_no = receipt_no + 1;
receipt_txt.Text = receipt_no.ToString();
}
protected void bind_monthyear()
{
DataSet ds2 = new DataSet();
ds2 = obj.fee_collections(3, DateTime.Now,0,0.0,0,0,"","",0,DateTime.Now,0);

if (ds2.Tables[0].Rows.Count > 0)
{
DataTable dt = ds2.Tables[0];
foreach (DataRow dr in ds2.Tables[0].Rows)
{
DateTime date1 = DateTime.Parse(dr["fee_for_month"].ToString());

date1 = date1.AddMonths(1);
ListItem month_list = new ListItem();
month_list.Text = date1.ToString("MMMM-yyyy");
month_list.Value = date1.ToString();
string test = date1.ToString();

DataColumn[] keyColumns = new DataColumn[1];
keyColumns[0] = dt.Columns["fee_for_month"];
dt.PrimaryKey = keyColumns;
if (ds2.Tables[0].Rows.Contains(test))
{

}
else
{
this.date_list.Items.Add(month_list);
}

}

date_list.SelectedIndex = date_list.Items.Count - 1;
date_list.SelectedValue = date_list.SelectedIndex.ToString();
var itemCount = date_list.Items.Count;
DateTime dn = Convert.ToDateTime(date_list.SelectedValue);
date_list.SelectedValue = null;
DateTime ds = dn;
for (int i = 0; i < 12 - itemCount; i++)
{
ds = ds.AddMonths(1);
ListItem month_list = new ListItem();
month_list.Text = ds.ToString("MMMM-yyyy");
month_list.Value = ds.ToString();
this.date_list.Items.Add(month_list);
}

}
else
{
DateTime dn = DateTime.Now;
for (int i = 0; i < 12; i++)
{

dn = dn.AddMonths(1);
ListItem month_list = new ListItem();
month_list.Text = dn.ToString("MMMM-yyyy");
month_list.Value = dn.ToString();
this.date_list.Items.Add(month_list);

}
}

}

protected void Onsave(object sender, EventArgs e)
{
int id = Convert.ToInt32(Session["id"]);
int sid = Convert.ToInt32(ViewState["s_id"]);
int fid=Convert.ToInt32(ViewState["f_id"]);
int bid = Convert.ToInt32(ViewState["b_id"]);
double amount = Convert.ToDouble(ViewState["amount"]);
string mode = cash_list.SelectedItem.Text.ToString();
string bname = bname_txt.Text.ToString();
string chq_no = cheque_txt.Text.ToString();
long cheque_no;

if (chq_no == "")
{
cheque_no = 0;
}
else {
cheque_no = long.Parse(chq_no, System.Globalization.CultureInfo.InvariantCulture);
}
string cdate = txtCalendar.Text.ToString();
DateTime chq_date ;

if (cdate=="")
{
chq_date = DateTime.MinValue;
}
else{
chq_date = Convert.ToDateTime(cdate);
}
foreach (ListItem item in date_list.Items)
{
if (item.Selected)
{
string selectedValue = item.Value;
int receipt_no = Convert.ToInt32(receipt_txt.Text);
DataSet ds1 = new DataSet();
ds1 = obj.fee_collections(2, Convert.ToDateTime(selectedValue), receipt_no, amount, sid, fid, mode, bname, cheque_no, chq_date,id);

}
}
receipt_txt.Text = "";
}
protected void onchecklist_change(object sender, EventArgs e)
{
double amount = Convert.ToDouble(ViewState["amount"]);

if (amount.ToString()=="" || amount.ToString() == null)
{
amount = 0;
}

var total = date_list.Items.Cast<listitem>().Where(Items => Items.Selected).Count();
double fee = amount * total;
total_txt.Text = fee.ToString();

}

protected void bind_collections()
{

int sid = Convert.ToInt32(ViewState["s_id"]);

DataSet ds6 = new DataSet();
ds6 = obj.fee_collections(4,DateTime.Now,0,0,sid,0,"","",0,DateTime.Now,0);

if(ds6.Tables[0].Rows.Count >0)
{
collection_grid.DataSource = ds6;
collection_grid.DataBind();
}
}
}