Double-spending means committing the same money twice: promising it to one person, and promising the same units to somebody else before anyone notices.
With physical cash it is impossible, and not because it is forbidden. A banknote is an object, and objects can only be in one place. When you hand one over, you no longer have it. Physics does the work.
Anything digital loses that property. Copying a file produces an identical file, and “sending” one is really duplicating it — the original stays with you. So the protection has to come from somewhere else.
It does not come from making money impossible to copy. It comes from a referee: an institution that holds the list and checks it before agreeing to change it. Which is why double-spending is not really a problem about copying at all. It happens wherever there is a gap before somebody checks — a cheque taking days to travel back in the 1980s, or a photograph of a cheque presented twice in two different places.