[v7 General Discussion] Problem with updating song changes in schedule

We have recently started using EW and I imported the 75 songs from SS that we have sung over the past 18 months. Following instruction on the site for copying data to a different computer I packaged up the relevant files to be distributed to worship leaders with a copy of the program so they could set up their own schedules.
All went well and this mornings schedule was imported and worked fine BUT when I used Check Schedule for Changes it wanted to import some songs as new even though they were already there. See pic for an example....

[attachment=0:2w0owlrx]Song Update.PNG[/attachment:2w0owlrx]

As you can see the song Higher+Wider+Deeper already exists on the Songs tab but the window only gives me the option to add, not update and when I did I then had 2 copies of the song. What identifies a song? As the song came from a copy of my original files, what makes it uniquely different? Or am I going about this the wrong way?
Songs imported from SS give you the words but not necessarily the proper structure of the song. I was expecting that as leaders set up schedules they would set the structure of songs which would then be imported as updates to existing songs and so over time the songs would end up in an "as written" state.

Can I have some direction/instruction please.