I have a fairly regular issue with users creating multiple accounts, whether it's to slip past a ban, lost password, or simple confusion.
To try to detect these, I've put together a script which lists every IP address that has seen more than one account post from it. (Script attached)
It's not pretty, and you will need to customize it a bit, but it should do the trick. Please note that this page takes some serious processing to build, so you may want to run it infrequently.
Each IP address is clickable, taking you to the forum "track IP" page for that address. If an account still exists, it's name will be clickable as well, taking you to the user's profile.
Note that since this page is not protected, you should either remove it when not in use, or place it under a password protected web directory. (It could allow users to get address info for each other, if they know you're using it.)
Enjoy, and good hunting!

Edit: Slightly updated, to allow easier setup. (DB/table names have been turned into variables.)
Edit 2: Updated to work with 1.1.
Edit 3: Updated for SMF 2... I can't un-attach the 1.1 version, so here is a link to the SMF 2 script:
http://army.ca/deleteme/multiple_accts_php.txt