GridView Button in the footer fires twice

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ASP.NET

Gridview

Hi Experts,
I have a the below GridView :

<asp:GridView runat="server" ID="SO_Grid" AutoGenerateColumns="false" DataSourceID="so_emails_DataSource" DataKeyNames="U_Email" CssClass="table table-hover table-striped" HeaderStyle-ForeColor="#10466E"
     ShowFooter="true"><%--OnRowDeleting="SO_Grid_RowDeleting"  OnRowCommand="SO_Grid_RowCommand"--%>
<EmptyDataTemplate>no emails found</EmptyDataTemplate>
 <Columns>
 <asp:TemplateField HeaderText="SO Email">
  <ItemTemplate>
    <asp:Label runat="server" ID="lbl_SO_Email"><%#Eval("U_Email") %></asp:Label></ItemTemplate>
  <FooterTemplate>
    <asp:TextBox runat="server" ID="txt_SO_Email"></asp:TextBox></FooterTemplate>
     </asp:TemplateField>
      <asp:TemplateField HeaderText="Delete Email">
     <ItemTemplate>
      <asp:Button runat="server" ID="btn_Delete" Text="Delete" CommandName="remove" ForeColor="#10466E" BackColor="Transparent" BorderStyle="None" OnClientClick="return confirm('Are you sure you want to delete this email?');" OnClick="btn_Delete_Click" CommandArgument='<%#Container.DataItemIndex %>' />
   </ItemTemplate>
   <FooterTemplate>
  <asp:Button runat="server" ID="btn_Add" Text="Add New" CommandName="newEmail" ForeColor="#10466E" BackColor="Transparent" BorderStyle="None" OnClick="btn_Add_Click" /></FooterTemplate>
      </asp:TemplateField>
    </Columns>
</asp:GridView> 

the Add New Email Button on the footer fires twice, it adds new email twice!
I tried to add the new email on RowCommand it does the same so I changed my code to add on Button click
Here is my code to add new Email:

protected void btn_Add_Click(object sender, EventArgs e)
        {
            TextBox new_SO_Email = (TextBox)SO_Grid.FooterRow.FindControl("txt_SO_Email");
            string email = new_SO_Email.Text;
            DataTable mytb = clsBP.Check_BP_Emails(lbl_BP_Id.Text, email, "SO");
            if (mytb.Rows.Count > 0)
            { new_SO_Email.Focus(); ClientScript.RegisterStartupScript(this.GetType(), "callfunction", "alert('This Email already exists.');window.location.href = BP.aspx;", true); }
            else
            {
                bool isAdded = false;
                isAdded = clsBP.Add_BP_Emails(lbl_BP_Id.Text, email, "SO");
                if (isAdded)
                {
                    SO_Grid.DataBind();
                    lbl_SO_Msg.Visible = true;
                    //ClientScript.RegisterStartupScript(this.GetType(), "callfunction", "alert('Thank you! The Email is successfuly deleted.');window.location.href = BP.aspx;", true);
                }
                else
                { ClientScript.RegisterStartupScript(this.GetType(), "callfunction", "alert('Database problem, please try again later.');window.location.href = BP.aspx;", true); }
                new_SO_Email.Text = "";
            }
        }

Please advise

Thanks in advance!