Same download code for all mp3 files on an album

1. create release 1 with mp3 file
2. create 30 download codes
3. create release 2 for another mp3 file
4. try to import the 30 download codes generated for release 1 on release 2
Now I get the error message that these download codes are already in this release

I thought this would be a good idea to use the same download codes for all mp3 files on an album. But download codes seems to be general and not connected to release.