Hi LarryPerson.
An Echo Nest song represents an acoustic entity. So many different Spotify tracks can map to a single Echo Nest song if the same audio was released multiple times (like on an album, a single, a compilation, etc). But different live recordings will certainly have different audio, and hence different SO IDs.
You can see this in action in the API by finding the list of songs called The Wheel by The Grateful Dead, and seeing how Spotify tracks map to them:
http://developer.echonest.com/api/v4/song/search?api_key=XXX&artist=The%20Grateful%20Dead&title=The%20Wheel&results=100&bucket=tracks&bucket=id:spotify
In this case, it looks like every different song has exactly one track. But compare that with, for example:
http://developer.echonest.com/api/v4/song/search?api_key=XXX&artist=Miley%20Cyrus&title=Wrecking%20Ball&results=100&bucket=tracks&bucket=id:spotify
-Eli