You may have two multilib definitions in pacman.conf (see
https://bbs.archlinux.org/viewtopic.php?id=191802).
Just a note that if you experience error messages then it's best to paste then into forums exactly as they are. In this case I think it would have been:
Quote:
could not register 'multilib' database (database already registered)
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This allows others to search the internet for that exact error string and help you out.
The link I posted above was found by searching on the internet for (the string made non-dependent on the actual database name):
pacman "could not register" "database already registered"