I just tested two Windows 10 devices, and after the first sync, no files are sent on subsequent synchronizations. So I'm definitely going to need either more information on how to reproduce this (create a new song and see if you can get it to exhibit the incorrect behavior), or a copy of library backups from two devices that are exhibiting the problem. The other thing that might help is if you export the .msf file for the song you already shared from the other device, so that I can import that copy of the song on my 2nd device, and then hopefully I'll see the issue.
Thanks,
Mike
Thanks,
Mike