in the chart shows the error

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C#

sql = "select distinct r.rate as Rate,f.bfid as BFid FROM bthfac AS f,batch b,facfeedback r";
sql = sql + " where month(b.examdate)= '" + Ddlmonth.Text + "' and year(b.examdate)= '" + DropDownList2.Text + "' and ";
sql = sql + " r.bfid=f.bfid and b.bthid=f.bthid and f.facid = '" + DropDownList4.SelectedValue + "'";
dr = scon.ReadSql(sql);
GridView3.DataSource = dr;
GridView3.DataBind();
GridView3.Visible = true;


//Displaying the gridview records in the chart for that code as follows

dr = scon.ReadSql(sql);
int[] LRate = new int[GridView3.Rows.Count];
int[] LBFid = new int[GridView3.Rows.Count];
int i = 0;
while (dr.Read())
{
LRate[i] = int.Parse(dr[0].ToString());
LBFid[i] = int.Parse(dr[1].ToString());
Chart1.Series.Add(LBFid[i].ToString());
i = i + 1;
}


in the run mode when i execute shows error as follows;


:A chart element with the name '65' already exists in the 'SeriesCollection'.

what is the problem in my above code.

please help me.


Note it is web application using asp.net using c#.

Regards
Narasiman P.