Some explicit metadata could specify that they are the same. Otherwise, you don't know. I suppose this problem with replicas is the reason you like to have a reference to the object instead of the object itself. An explicitly declared equality relationship is another way. An operation for determining equality is another way.
Same identifier is yet another way.
But I think that we will have to learn to live with multiple identifiers for the same object. You could then compare what two different identifiers refer to by comparing their preferred identifiers. Yes, URLs and resources are exactly 1-1, in both directions. But, of course, the claim I made does not mean 1-1.