I don't know what to say about Duet that hasn't been said already. It deserves its plaudits, as it's an episode that almost reduced me to tears when I first saw it. It gets me every
time I rewatch. It's brilliant!
You have your Trekkian tolerance-type special episode, but it is merely the cover of the mystery that is enthralling. The twist is one I just wasn't expecting, and then of course the ending leaves me crippled.
This is the most definite hint yet of the type of story DS9 would later revel in.