An item with the same key has already been added radgrid

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ASP.NET

C#3.5

Telerik

Hi,
I am developing an application using Rad Controls.In this there is a rad grid which contains checkbox,on selection of it based on the id i am loading relevant data to the another radGrid next to it in the same page.
For the first time selection data is getting loaded properly,But on the next checkbox selection i am getting the error an item with the same key has already been added.

Sharing the code

<telerik:RadGrid ID="radGrid1" AutoGenerateColumns="false" AllowPaging="true"  runat="server"
                ShowHeader="true" AllowAutomaticUpdates="true" AllowAutomaticInserts="true" AllowAutomaticDeletes="true"
                Width="43%" Style="float: left;" Visible="true" >
                <MasterTableView DataKeyNames="USERNAME,USERID">
                    <Columns>
                        <telerik:GridTemplateColumn>
                            <ItemTemplate>
                                <asp:CheckBox ID="UserSelect" runat="server" HeaderText="Select" AutoPostBack="true"
                                    OnCheckedChanged="UserSelect_CheckedChanged" />
                            </ItemTemplate>
                        </telerik:GridTemplateColumn>
                        <telerik:GridBoundColumn UniqueName="USERID" DataField="USERID" HeaderText="User Id"
                            Visible="false">
                        </telerik:GridBoundColumn>
                        <telerik:GridBoundColumn UniqueName="USERNAME" DataField="USERNAME" HeaderText="User Name">
                        </telerik:GridBoundColumn>
                    </Columns>
                </MasterTableView>
            </telerik:RadGrid>

Checkbox change event

protected void UserSelect_CheckedChanged(object sender, EventArgs e)
{
    CheckBox chk = (CheckBox)sender;
    GridDataItem itm = (GridDataItem)chk.NamingContainer;
    RadRoles.DataSource = new string[] { };
    RadRoles.DataBind();

    foreach (GridDataItem row in radGrid1.MasterTableView.Items)
    {
        CheckBox chk1 = (CheckBox)row.FindControl("UserSelect");

        if (chk1.Checked == true)
        {
            DataTable dt = null;
            //dt will fetch the values from the DB
            RadRoles.DataSource = dt.DefaultView;
            RadRoles.DataBind();
        }
    }
}