I used to duplicate a sold model incorporating the adjacent teeth and preps in one go, it works ok but fitting the veneers down to the die can be a pain especially if the prep wraps round the contacts, multi veneer cases were also a difficult when it came to sectioning prior to glaze, also the mass of refractory can lead to underfiring and bubbles.
These days I use a simple model tray from Tricodent (not sure if these are available in the US, I'm sure you can find an equivalent though) I saw out the die and then trim the margins prior to duplicating just the die, 9 times out of ten the refractory seats back in tray with no problems. Building up is easy, as is trimming and contact adjustment, best of all if I have a case with crowns and veneers together I can build up in one go. It's very effective and simple.
I also use the Giroform system which has a duplicating flask and special heat proof pins, this works ok but the pins are quite fragile and it can be more of a faff.
