If you, as administrator, edit a user profile (Administration > Users > Manage users) you may get an on-screen warning like:
Duplicate email address. A user with that email already exists.
This should only happen if you are changing a user’s email address and using one that another user already has, but is currently appearing for any action, even including saving the record after making no change at all.
This does not cause any problems with the function of the site, but can be annoying. It is fixed for the next release (3.3.9), but you can download the fix now if you wish. Just download the zip folder attached to this post, extract the file ‘admin_users.php’ and upload it your your web server, replacing the file of the same name in the kiwitrees root folder.