A doppio is always two shots of espresso poured into one cup. From the usual portion of espresso (shot) doppio differs only in volume and twice the amount of caffeine in the composition.
In terms of volume, doppio coffee is similar to lungo. However, in order for a lungo to be twice the size of an espresso shot, coffee is used with a coarser grind, or in rare cases, the extraction time is doubled. Doppio is made from the coffee of the same degree of grinding as espresso.
Another black coffee that is served in large portions is the Americano. But it is espresso diluted with water. Doppio is not diluted.
Doppio differs from coffee brewed in a cezve, in a geyser coffee maker, French press, or pour-over in that it can only be prepared in a coffee machine or double-holder carob coffee maker.