Not all SodaStream Carbonating bottles are compatible with all SodaStream Sparkling Water Makers. For example, with the Crystal you can only use the 0.8L glass carafe.
The DUO is specially designed to fit the 1L glass carafes and 1L Fuse bottles, and therefore has a special flask that only fits a certain size of bottles (height and width).
Keep in mind the DUO is compatible with our existing 1L Fuse bottles.