It's the wrong use of the metaphor.
Newcastle is (was?) a mining town, so 'carrying coals to Newcastle' means giving as a gift something the recipient already has in abundance.
There are numerous metaphors for doing something futile:
"Like trying to poke a cat out from under a porch with a wet rope" Comes to mind.
Also: "teaching an old dog new tricks" or "A leopard cannot change his spots". I'm sure others will chime in...
Interested in Language