C# ASP.NET, SQL Server : Button Click inserts duplicate rows

I have a registration form where on button click a stored procedure in a SQL Server is called and a new row is inserted. The problem is when I click (no accidental double clicks) two rows of the same entry is inserted. I have gone through my codes thoroughly but I could not find the issue.

C# button click code:

protected void Button1_Click(object sender, EventArgs e)
{
    int userId = 0;
    string constr = ConfigurationManager.ConnectionStrings["islamguiderConnectionString"].ConnectionString;

    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand insertUser = new SqlCommand("Register_Student"))
        {  
            insertUser.CommandType = CommandType.StoredProcedure;

            insertUser.Parameters.AddWithValue("@StudentName", TextBox1.Text.ToUpper());
            insertUser.Parameters.AddWithValue("@Email", TextBox4.Text.ToLower());
            insertUser.Parameters.AddWithValue("@CourseCode", DropDownList2.SelectedValue);
            insertUser.Parameters.AddWithValue("@CourseTitle", DropDownList2.SelectedItem.ToString());

            insertUser.Connection = con;

            con.Open();
            insertUser.ExecuteNonQuery();

            userId = Convert.ToInt32(insertUser.ExecuteScalar());
            Con.Close()
        }
    }
}

My SQL Server stored procedure:

CREATE PROCEDURE [dbo].[Register_Student]
    @StudentName NVARCHAR(200),
    @Email NVARCHAR(200),
    @CourseCode NVARCHAR(200),
    @CourseTitle NVARCHAR(200)
AS
BEGIN
    SET NOCOUNT ON;
    DECLARE @ID INT 

    INSERT INTO dbo.Registration_Table (StudentName,Email, CourseCode, CourseTitle) 
    VALUES (@StudentName, @Email, @CourseCode, @CourseTitle)  

    SELECT 2
END