Reproduction:
(hint: I use the german translation and thus have to translate everything back to english for the bug report. So please forgive me, if not all terms are exactly like they would be in an English phpBB)

1. Login to Board
2. Click your "Username", then "Profile"
3. Click "Change Profile"
4. Alter your date of birth
5. Click "Submit"
=> MySQL Error:
Duplicate entry '2' for key 'PRIMARY' [1062]

For the exact result please see the screenshot. I already tried this with different users and I tried to repair the database.

What seems strange to me ist that phpBB tries an INSERT where I only want to alter my profile data.

Maybe this is related? http://stackoverflow.com/questions/6153552/mysql-error-1062-duplicate-entry-for-key-2

Many thanks for any help!