If you want to remove the word "duplicates":
string duplicatesRemoved = RTBstring.Replace("duplicates", "");
;)
The easy (and overly simplistic) way to remove duplicate words is to split on the space character and use LINQ's Distinct() method:
string duplicatesRemoved = string.Join(" ", RTBstring.Split(' ').Distinct());
But this won't work in a useful way if you're working with actual sentences (i.e. punctuation will break it). Without a clear definition of what you mean by duplicates and what the expected input is, it's hard to give an accurate answer.
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