in reply to Replacing with multiple occurrences.
s/((.)\2*)\1$/$1/; [download]
This wasn't tested, I assume, as it doesn't work as requested. Try it with my string below.
Update: To clarify: 'aaabcdabcddddddefgg' would become 'aaabcdabcddddddefg' using your version.
Update 2: Oops. I misread the question as 'all' duplicates in the string. Thanks to physi for pointing out my fault. Interesting exercise, anyway.