Hi rexestremera,
The song search API doesn't have a unique artist filter. The reason you see what look like duplicates is that there really are more than one version of many songs with different audio and therefore different SOng IDS. There are no 'cultural song' entities in the API (where there would be just one thing called Killing In The Name regardless of different audio), so you'll have to filter yourself on the client.
If you say more about what you're trying to do, we may be able to give you some more specific advice.
-Eli