[Tanjamuse]

Ok.

There's something else I don't understand,

why in one library is this the correct search:

SELECT book  FROM books_custom_column_32_link
 WHERE value IN
 ( SELECT value 
 FROM books_custom_column_32_link
 GROUP BY value 
 HAVING COUNT(*) > 1 )

and in another it's this:

SELECT book  FROM books_custom_column_9_link
 WHERE value IN
 ( SELECT value 
 FROM books_custom_column_9_link
 GROUP BY value 
 HAVING COUNT(*) > 1 )

When it's the same custom column I need to search?