I don't think it's behaving any differently than any other email application that I've used. If you Reply to a Reply, that will show as RE: RE:, which is exactly what it represents. This certainly isn't exclusive to gmail.
I actually thought gmail was the only service that DOESN'T keep adding the RE because its threaded. I don't recall it adding the re when I reply, it just shows one re in the title. Maybe I'm missing something...