You can check by doing something like:
(This example is in c#, but you should get the point)
if (!TextBox1.Text.Contains(value_to_be_inserted))
{
TextBox1.Text += value_to_be_inserted;
}
Beware though, this assumes that strings to be added will not be partial strings of strings already contained in the text box. To check for that, you will probably need to use a delimiter: white space, comma, etc, whatever works according to your data. Or if needed, you could store each string as you add them into a list and check against that.
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