The same reference (with the same hash) can be used in multiple statements, even across entities. For example, “imported from: English Wikipedia”, wdref:fa278ebfc458360e5aed63d5058cca83c46134f1 in RDF, is used in twelve million statements. As far as I’m aware, there is no constraint that cares where in a reference a snak is used (though there are constraints that care whether it is used in a reference!), so we should be able to check constraints on reference snaks only once, and reuse the result for all other places where the same reference occurs.