The legal relationship needn't last forever.

That is, I'd support a pair of copies legally filing for the equivalent of divorce. And just as with divorce, the couple needs to work out a division of assets at that point, perhaps with the "assistance" of a court of law, or a professional arbitrator.

Of course, if they signed a pre-duplication contract, that's fine too, just as with divorcing couples today.

Incidentally, as a matter of law I'd expect a pre-duplication contract to be binding on both parties X and Y if and only if the law recognizes both X and Y as the original. If copy X is not the original, legally speaking, then copy X was not a signatory to that contract and is not bound by it.