The install script for 1.9 fails on our system when it tries to had this index.
The CREATE UNIQUE INDEX statement terminated because a duplicate key was found for the object name 'dbo.mdl_course_modules' and the index name 'mdl_courmodu_idn_uix'. The duplicate key value is (<NULL>)
There is a idnumber column in the mdl_course_modules field, but it's not being used. All the values are NULL and therefore they aren't unique.