Prevent spoken messages from being replayed in a chat application.

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C#

I am making a LAN chat application with spoken messages using C#. I'm using SAPI 5.1, and I have encountered problem with speaking the messages. If I write the message "HI" his computer will receive the message and speak the word "HI" which is OK, but if I write a new message for example "HELLO" his computer will repeat speaking the first message "HI" and then speak the word "HELLO".

The old message "HI" should not be repeated again, and it should say the new message "HELLO".

Here is my code:

private void rtbConversation_TextChanged(object sender, EventArgs e)
{
    speech.Rate = speechRate;
    speech.Speak(rtbConversation.Text, SpeechVoiceSpeakFlags.SVSFlagsAsync);

    speech.Voice = speech.GetVoices(string.Empty, 
                          string.Empty).Item(combo1.SelectedIndex); 
}