MessageBox show twice c#

Question

I don't have any idea why is messagebox appearing twice in exit event Here's my code:

private void dashboard_FormClosing(object sender, FormClosingEventArgs e)
    {
        DialogResult dr = new DialogResult();
        dr = MessageBox.Show("Are you sure you want to close the application?", "Message", MessageBoxButtons.YesNo, MessageBoxIcon.Question);
        if (dr == DialogResult.Yes)
        {
            ConnectionString.conn.Open();
            SqlCommand cmd = new SqlCommand("delete from user_session_tbl where SESSION_ID = '" + Global_Variables.id + "'", ConnectionString.conn);
            cmd.ExecuteNonQuery();
            ConnectionString.conn.Close();
            MessageBox.Show("Goodbye " + Global_Variables.fname + " Have a nice Day!", "Message", MessageBoxButtons.OK, MessageBoxIcon.None);
            Application.Exit();
        }
    }