If I replicate a preset (not copy) - are all of its voices digitally duplicated, too?
No. Only the voice parameter sections are cloned, and a preset parameter section is added.
The samples themselves are still shared.
So this would only require a few hundred additional bytes of parameter data...
Note that truncation settings (and of course also loop settings) are stored as voice parameters; so if the sample to be truncated in Voice1/Preset1 is also used by another voice (Voice2/Preset2) which doesn't need the truncation, then the EII is actually capable of dealing with that without having to clone the sample or without sharing the truncation for all voices where the sample is used.
The data model design of the EII banks is messy, but there are actually some smart parts in it
(if I remember well, the Emax is not that advanced with this kind of stuff...)