C# using streamwriter to save the text while typing

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C#

this is my code

private void richTextBox1_KeyDown(object sender, KeyEventArgs e)
        {


            if (Properties.Settings.Default.url == "")
            {

            }
            else
            {
                if (File.Exists(Properties.Settings.Default.url))
                {
                    using (StreamWriter writer = new StreamWriter(Properties.Settings.Default.url, true))
                    {
                        writer.Write(richTextBox1.Text);
                        writer.Close();
                        Properties.Settings.Default.Save();

                    }
                }
                else
                {
                    MessageBox.Show("File Path: " + Properties.Settings.Default.url + " Dose Not Exist");
                }
            }



        }

this just makes sure the file path is there, and it also will write the text to that document, but my problem is that, when it writes the text it really stuffs up the text file for example


instead of the test being "Hello"

and me adding the text " Sup"

it should like this "Hello Sup"

but it would look something like this "HelloHello S HelloSu HelloSup Hello Sup"

it looks very odd i was wondering if anyone can help?