In a true double impression, the whole stamp is clearly doubled, like this:

from Bhopal State.
There are other ways you can have partial doubling of the impression: a slip print, where the paper shifts, in intaglio printing; a kiss print, where the plate bounces slightly, in letterpress or typography, and blanket offsets in offset-lithography, where the rubber blanket with the impression flaps against the paper. The catalogues don't usually list them.