Error on C# Web App I'm building

I'm getting an error:

The variable name '@GCSSalesPerson' has already been declared. Variable names must be unique within a query batch or stored procedure.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;

public partial class Customer : System.Web.UI.Page
{
    protected void
Page_Load(object sender, System.EventArgs e)
    {
        lblErrMsg.Visible = true;
    }
    protected void btnInsert_Click1(object sender, EventArgs e)
    {
        SqlConnection thisConnection = new SqlConnection(ConfigurationManager.ConnectionStrings["GSI2ConnectionString"].ConnectionString);

        //Create Command object
        SqlCommand nonqueryCommand = thisConnection.CreateCommand();

        try
        {
            // Open Connection
            thisConnection.Open();

            // Create INSERT statement with named parameters
            nonqueryCommand.CommandText = "INSERT  INTO Customers
(ChurchName,ContactName,ContactPhoneNumber,ContactCellNumber,ContactEmail,
FaxNumber,ChurchAddress,ChurchAddress2,ChurchCity,ChurchState,ChurchZipCode,
Notes,GCSSalesPerson) 
VALUES 
(@ChurchName,@ContactName,@Cont开发者_Python百科actPhoneNumber,@ContactCellNumber,@ContactEmail,
@FaxNumber,@ChurchAddress,@ChurchAddress2,@ChurchCity,@ChurchState,@ChurchZipCode,
@Notes,@GCSSalesPerson)";


            // Add Parameters to Command Parameters collection
            nonqueryCommand.Parameters.Add("@ChurchName", System.Data.SqlDbType.VarChar, 70);
            nonqueryCommand.Parameters.Add("@ContactName", System.Data.SqlDbType.VarChar, 70);
            nonqueryCommand.Parameters.Add("@ContactPhoneNumber", System.Data.SqlDbType.VarChar, 12);
            nonqueryCommand.Parameters.Add("@ContactCellNumber", System.Data.SqlDbType.VarChar, 12);
            nonqueryCommand.Parameters.Add("@ContactEmail", System.Data.SqlDbType.VarChar, 50);
            nonqueryCommand.Parameters.Add("@FaxNumber", System.Data.SqlDbType.VarChar, 12);
            nonqueryCommand.Parameters.Add("@ChurchAddress", System.Data.SqlDbType.VarChar, 70);
            nonqueryCommand.Parameters.Add("@ChurchAddress2", System.Data.SqlDbType.VarChar, 50);
            nonqueryCommand.Parameters.Add("@ChurchCity", System.Data.SqlDbType.VarChar, 50);
            nonqueryCommand.Parameters.Add("@ChurchState", System.Data.SqlDbType.VarChar, 3);
            nonqueryCommand.Parameters.Add("@ChurchZipCode", System.Data.SqlDbType.VarChar, 6);
            nonqueryCommand.Parameters.Add("@Notes", System.Data.SqlDbType.VarChar, 7800);
            nonqueryCommand.Parameters.Add("@ContactEmail", System.Data.SqlDbType.VarChar, 50);
            nonqueryCommand.Parameters.Add("@GCSSalesPerson", System.Data.SqlDbType.VarChar, 50);

            nonqueryCommand.Parameters["@ChurchName"].Value = TextBox1.Text;
            nonqueryCommand.Parameters["@ContactName"].Value = TextBox2.Text;
            nonqueryCommand.Parameters["@ContactPhoneNumber"].Value = TextBox3.Text;
            nonqueryCommand.Parameters["@ContactCellNumber"].Value = TextBox4.Text;
            nonqueryCommand.Parameters["@ContactEmail"].Value = TextBox5.Text;
            nonqueryCommand.Parameters["@FaxNumber"].Value = TextBox6.Text;
            nonqueryCommand.Parameters["@ChurchAddress"].Value = TextBox7.Text;
            nonqueryCommand.Parameters["@ChurchAddress2"].Value = TextBox8.Text;
            nonqueryCommand.Parameters["@ChurchCity"].Value = TextBox9.Text;
            nonqueryCommand.Parameters["@ChurchState"].Value = DropDownList1.Text;
            nonqueryCommand.Parameters["@ChurchZipCode"].Value = TextBox11.Text;
            // nonqueryCommand.Parameters["@Notes"].Value = TextArea1.Text;
            nonqueryCommand.Parameters["@GCSSalesPerson"].Value = DropDownList2.Text;


            nonqueryCommand.ExecuteNonQuery();
        }

        catch (SqlException ex)
        {
            // Display error
            lblErrMsg.Text = ex.ToString();
            lblErrMsg.Visible = true;
        }

        finally
        {
            // Close Connection
            TextBox1.Text = "";
            TextBox2.Text = "";
            TextBox3.Text = "";
            TextBox4.Text = "";
            TextBox5.Text = "";
            TextBox6.Text = "";
            TextBox7.Text = "";
            TextBox8.Text = "";
            TextBox9.Text = "";
            TextBox11.Text = "";

            thisConnection.Close();

        }
    }
}

Are you sure the error message doesn't specify @ContactEmail?

You are adding @ContactEmail twice as part of your nonqueryCommand:

  nonqueryCommand.Parameters.Add("@ContactEmail", System.Data.SqlDbType.VarChar, 50);

This would exactly match that error message.

you are specifying @ContactEmail twice in your query and also in adding commands

nonqueryCommand.Parameters["@ContactEmail"].Value = TextBox5.Text;