The second time I try to import the same LDIF I always get the error "Selected logfile already exists. Select option 'Overwrite existing logfile' if you want to overwrite the logfile." Therefore the "Finish" button is disabled. Neither selecting 'Overwrite existing logfile' nor selecting "Use custom logfile" with another name does disable this error. The only workaround is to restart Directory Studio.
To reproduce just try to import the same LDIF twice into an embedded server with Directory Studio under Mac OS X