The Readymade is an obvious candidate.
In fact, the “original” Bottle Dryer was lost, and all we have now are later versions. Here is a “replica” that Duchamp baptized in 1959:
It should also be possible to make copies of Dan Flavin’s sculptures that are as good as the originals.
We shouldn’t assume that only an exact copy could be as good as the original. Arguably, this very image of Andy Warhol’s painting conveys the essentials of the work itself.
The view I’m advancing assumes that a work’s formal features are all that matter to an assessment of its quality. Perhaps that’s not the case, or not always the case. If we have cognitive as well as aesthetic interests in artworks, it makes a difference whether what we’re experiencing is personally connected to its maker. The original and the copy are different things with different statuses, even if they’re visually identical.