How can I sync even though Mac says duplicate library

I last synced my iPad about 2 months ago with my MacAir. I now wanted to do this again, but all of a sudden the system claims that my iPad has been synced with a different library. When I check in the preferences where the library resides, it is still the same one. I can't find this 'duplicate' library at all.

I don't want to sync now as the system claims all my content on the iPad will be erased. There are files on there I need to keep.

How do I solve this so that iTunes syncs with and keeps the files on my iPad?