You're not alone. I get two emails too.
I see this on a lot of phpbb forums where I've logged in and modified my account details (avatar, sig, etc) before posting the first time.
A long time ago, I had the same problem with a phpbb board I was hosting, and when I upgraded, apparently one of the user reply tables got all its email recipient records duplicated in another table if the user had subscribed to more than one topic in the same forum. I haven't used phpbb in a couple of years now, but that's one of the problems I've seen.
Sometimes it's a language thing too - if you sign up for non-standard (non-US English) languages, OR if you enter a different timezone to the default, that can trigger the same behaviour. I know that because I did the same thing.
There are any number of other forums that have the same problem, at least with me. So when I join, I make sure I post (if they have a sandbox or a newbie-say-hi thread) first, and only then I update my details. Sometimes it works, but sometimes not. The older the forum (or the less-up-to-date it is) the more likely it is to start getting duplicates.
Dunno if this helps. But yeah, it's really annoying, and actually, it's not that hard to fix. But if the forum admin doesn't have the time to dive into phpbb guts, it will not go away, even with the new updates.
[EDIT] Oops, posted twice due to a phpbb server error. Seems to be OK now. Hmmm. Wonder if it's related?
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