In a normal Rebuild game, you vie for Duchies to both get the most points immediately and the most options for 3VP Rebuild plays (Duchy -> Province). In these games any Estate that remains after the Duchies are gone is a liability because it decreases the expected return of your Rebuilds by occasionally being forced to Rebuild Estate -> Estate for a 0VP gain.
In a Duke game, however, those Estates remain useful to the point that you want to get alot of them, as even splitting the Duchies now allows for a 3VP gain by Rebuilding Estate -> Duke, which is as good as Duchy -> Province in a normal Rebuild game. As a consequence, in Duke games you actually want as few Provinces as possible in your deck, as here Province serves the same role as Estates in a normal game: your inability to protect them decreases the expectation of your Rebuild plays. However, beyond not discarding Dukes with sifters or in the face of a discard attack, there's very little you can do to avoid gaining Provinces because Duchies should always be protected over Dukes.
Rebuild-Duke games usually end with a Duchy/Estate/Duke 3-pile, in that order.
