I've just run the plugin Find Duplicates and no duplicates matching both book title and author where found.
Burt now, when I click on an author, instead of just showing the author's books, it shows all books but with the author's books highligted in green.
I've restarted calibre but it didn't help. I've been through Preferences but can't seem to find how to revert to the previous functionality: clicking on an author and just showing this author's books.
Edit: Sorry guys. Just found it under Preferences -> Searching -> Highlight Search results...
I guess the Find Duplicates plugin turned that option on.