Song duplicates added when using different author types for the same author

Hello,

arch linux, trunk, r2371, also first spotted in r2371

1) Create a song
2) Assign the author "test test" with the type "words" to the song
3) Assign the author "test test" with the type "translation" to the song
4) Add a verse and title
5) Save the song and add it to the service
6) Save the service and reopen the service.

RESULT:
A duplicated song is added.