That's a bit of a tangle... there are a couple of ways to synchronize a rekordbox library -- from easiest to most difficult:
- The rekordbox Creative plan Cloud Sync feature; this automatically keeps each computer up-to-date using the cloud server for database and Dropbox for the files.
- An external hard drive; keep your music and database on an external drive, connect it to whichever computer you want to use and it's always in sync.
- The unofficial / unsupported Dropbox / Google Drive sync; basically, you manually move your rekordbox database to a folder within the cloud sync folders, and modify the rekordbox settings files on each computer to point to that location. This requires that you have the same paths for the user profiles on each machine.
- Pick one computer as the master and backup the database, then restore on the other. It's a one-way sync, but allows you to take the data to the other computer. Can work with different folder paths, but not ideal.
Don't try and use the XML import, you're just going to make a mess (as you've noticed with the warning about files existing in the collection).