When I right-clicked on a HTML file in Finder and chose “Open With”, there were 3 entries for Opera. This command (found on Apple support forums) fixed it:

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain user

This should rebuild your launch services database, which removes the duplicate entries.