There are 36 include's in the includes/acp files, such as this one (line 31 in include/acp/acp_ban.php):
include($phpbb_root_path . 'includes/functions_user.' . $phpEx);
which isn't guarded by function_exists, as on line 1865 in includes/function_convert.php:
if (!function_exists('user_add')), wh
Even though my analysis is based on 3.0.8 I just checked and the problem is still present in 3.0.10 RC3.
That these includes are not guarded raises "Cannot redeclare..." as function_user.php is very useful to include, if one would want to implement an new authentification mechanism (such as auth_apache.php).
Either these includes should be changed to include_once or they should be guarded by function_exists. I would go for include_once, but then I guess all of the includes in the repository should be changed to include_once for consistency.
See http://area51.phpbb.com/phpBB/viewtopic.php?f=99&t=42417 for a discussion on the issue.