Folks,
In addition to what Paul noted, we constantly get new versions of songs with the same name (e.g. live versions, remixes, extended or shortened). These will usually have different audio attributes from the "original". Take for example the Mumford & Songs song "I Will Wait":
http://developer.echonest.com/api/v4/song/search?api_key=API_KEY&artist=Mumford%20&title=I%20Will%20Wait&bucket=audio_summary
You can often use other filters like song_category or duration to find a particular version.
= David =