remove duplicate sentences between specified words from string

Question

This is my example of some string:

1. message: this is some message 2. message: this is some message 3. message: this is message which i want to see only in results

according to this example I would like to get result:

this is message which i want to see only in results

there might be multiple x. message: sentences in my string.

I want to delete those message which are duplicated

any idea?

ADDED:

My question is regarding some selling tool and printing reports. Code I running now looks like this:

private void OnBeforePrint(object sender, System.Drawing.Printing.PrintEventArgs e) {
    string a = xtraReport1.GetCurrentColumnValue("Paczka_Notatki").ToString();
    xrTableCell3.Text = a;
}

when I change it to:

private void OnBeforePrint(object sender, System.Drawing.Printing.PrintEventArgs e) {
    var str = xtraReport1.GetCurrentColumnValue("Paczka_Notatki").ToString();
    foreach (var message in Regex.Split(str, @"\d+\. message: ")
        .GroupBy(m => m)
        .Where(m => m.Count() == 1 && m.Key != string.Empty)
        .Select(m => new { message = m.Key }))
        {
            xrTableCell3.Text = message.message;
        }
}

then I have got a lot of errors like:

Answer 1

String text = xtraReport1.GetCurrentColumnValue("Paczka_Notatki").ToString();

    string finalNote = "";
    string[] notes = System.Text.RegularExpressions.Regex.Split(text, @"\d{1,}\.\smessage:(?<Message>[\D\s]+)");

    foreach (string note in notes)
    {
        if (!String.IsNullOrEmpty(note) && finalNote.IndexOf(note) == -1)
        finalNote += note;
        }
    xrTableCell3.Text = finalNote;

thank you guys for support !!!