Regular expression to find and remove duplicate words

Question

Using regular expressions in C#, is there any way to find and remove duplicate words or symbols in a string containing a variety of words and symbols?

Ex.

Answer 1

Well, Jeff has shown me how to use the magic of in-expression backreferences and the global modifier to make this one happen, so my original answer is inoperative. You should all go vote for Jeff's answer. However, for posterity I'll note that there's a tricky little regex engine sensitivity issue in this one, and if you were using Perl-flavored regex, you would need to do this:

\b(\S+)\b(?=.*\b\1\b.*)

instead of Jeff's answer, because C#'s regex will effectively capture \b in \1 but PCRE will not.