donot insert duplicate values in database when added from gridview footer textboxes

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C#

ASP.NET

SQL-Server

I am working on asp.net c# sqlserver 2005 database.

on my webpage i have a gridview, in this gridview i have 4 fields (Username,Password, email,designation) and 4 textboxes in footer to enter the records. and a add button to add the record.

if the record already exists in database, display a message Username already exists.

Thanks in advance.

This is my code in row command in gridview.

protected void GVRegistrations_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName.Equals("AddNew Registrations"))
        {
 TextBox TxtUsername  = (TextBox)GVRegistrations.FooterRow.FindControl("TxtFooterUsername");

 TextBox TxtPassword = (TextBox)GVRegistrations.FooterRow.FindControl("TxtFooterPassword");

            TextBox TxtEmail = (TextBox)GVRegistrations.FooterRow.FindControl("TxtFooterEmail");

TextBox TxtDesignation = (TextBox)GVRegistrations.FooterRow.FindControl("TxtFooterDesignation");

            con.Open();
            SqlCommand cmd = new SqlCommand("INSERT into ACRegistrations(Username,Password,Email,Designation) values('" + TxtUsername.Text + "','" + TxtPassword.Text + "','" + TxtEmail.Text + "','" + TxtDesignation.Text + "')", con);
            int result = cmd.ExecuteNonQuery();
            con.Close();
            if (result == 1)
            {
                BindRegistrations();
                ScriptManager.RegisterStartupScript(this, this.GetType(), "RunCode", "javascript:alert('Record Added successfully');", true);
            }
            
        }
    }

here just i want to add to code, that if username already exists. if i enter the same username onceagain.