[exim] Regex String Help

OK - I don't know if this can be done but suppose I have a string with a
number of items in it that are separated by spaces - and I want to
remove all duplicate items for the string. Is there an easy way to do that?

Example string:

X-Sender-Nameserver: ns1.libero.it ns2.libero.it - ns1.libero.it ns2.libero.it - mx2.libero.it mx3.libero.it mx4.libero.it mx5.libero.it mx1.libero.it - mx2.libero.it mx3.libero.it mx4.libero.it mx5.libero.it mx1.libero.it - -