As reported:

when I try to create a user account on the Debian wiki which uses a
name already taken, I get the following message:

This user name already belongs to somebody else. If this is a new
account and you need another verification link, try sending another
one.

"sending another one" links to
https://wiki.debian.org/InstallingDebianOn/Allwinner?action=newaccount&i=1162379783.5.60584&resend=1
in my case. Going to this page results in an internal server error.

I've confirmed this and I can see the problem in the apache logs:

 [wsgi:error] mod_wsgi (pid=19132): Exception occurred processing WSGI script '/srv/wiki.debian.org/bin/moin.wsgi'.
 [wsgi:error]  Traceback (most recent call last):
 [wsgi:error]    File "/usr/lib/python2.7/dist-packages/werkzeug/wsgi.py", line 588, in __call__
 [wsgi:error]      return self.app(environ, start_response)
 [wsgi:error]    File "/usr/lib/python2.7/dist-packages/MoinMoin/wsgiapp.py", line 264, in __call__
 [wsgi:error]      response = run(context)
 [wsgi:error]    File "/usr/lib/python2.7/dist-packages/MoinMoin/wsgiapp.py", line 89, in run
 [wsgi:error]      response = dispatch(request, context, action_name)
 [wsgi:error]    File "/usr/lib/python2.7/dist-packages/MoinMoin/wsgiapp.py", line 137, in dispatch
 [wsgi:error]      response = handle_action(context, pagename, action_name)
 [wsgi:error]    File "/usr/lib/python2.7/dist-packages/MoinMoin/wsgiapp.py", line 203, in handle_action
 [wsgi:error]      handler(context.page.page_name, context)
 [wsgi:error]    File "/usr/lib/python2.7/dist-packages/MoinMoin/action/newaccount.py", line 259, in execute
 [wsgi:error]      mailok, msg = _send_verification_mail(request, theuser)
 [wsgi:error]    File "/usr/lib/python2.7/dist-packages/MoinMoin/action/newaccount.py", line 22, in _send_verification_mail
 [wsgi:error]      'v': user.account_verification}
 [wsgi:error]  AttributeError: User instance has no attribute 'account_verification'

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[..]

Currently this works; questionable if it should and if it really
should leak the user's email address. But it's not completely
broken.

Given it seems unlikely this will be fixed on MoinMoin, I'm gonna
close this and we shall hope the MediaWiki-migration will be
completed.

Best,
Chris