The belief in spirits is pretty strong in Duckburg, especially the belief in ghosts. Peg Leg Pete pretends to be a ghost in Barks' first duck story "Pirate Gold" (OS 9-02).

We have all reason to believe that this belief started with Uncle Scrooge. In "The Life and Times of Scrooge McDuck" Scrooge has several encounters with the late Sir Quackly. In part one (D 91308)he tells Scrooge about the history of the McDuck clan while he makes Scrooge believe the two are not related. Later Scrooge's horse impersonates a ghost, and scares away the Whiskervilles by making his horse.
In part five (D 92191) Sir Quackly helps Scrooge fight the Whiskervilles. Scrooge engages one of the Whiskervilles, and it looks like the Whiskerville is going to win when Sir Quackly hands Scrooge a sword. The sword is hit by a lightning, and Scrooge ends up in the McDuck heaven (or possibly their hell as they have to pay rent for the golf course). Scrooge's ancestors give him a second chance when they see his being such a miser (This story is, btw, an example of ancestral worship in the world of the Ducks). at the end of the story we see another ghost; Scrooge's mother.

Scrooge has forgotten all about this when he goes back to Scotland in "The Old Castle's Secret" (OS 189-02). In this story, a villain scares the ducks with a substance that makes one invisible. The "ghost" is revealed to be a villain related to Scrooge. Yet, it may be this episode which led to the Huey, Dewey and Louie's belief in ghosts. Usually they are more rational, while Donald displays this belief much earlier (we do not know why the belief in ghosts leads to rheumatism. Research will hopefully lead to a more precise diagnosis).

So, the belief in ghosts is pretty strong in Duckburg. We can see this in "The Strange Shipwrecks" (US 23-02). It is not a big secret that Donald and Scrooge believe in ghosts, but it is more of a surprise that Scrooge's captain and the smartest, bravest and most fearless detective are so scared that "their hair is coming through their caps." They think they have seen a ghost; the truth being that it is the Beagle boys who are using people's belief in ghosts to sack Scrooge's ships. The Beagle boys need to sack one more ship to make profit. It seems they have paid the equipment this time, which the did not do when they built The Paul Bunyan Machine (US 28-01).
Scrooge's strong belief in ghost is demonstrated in "House of Haunts" (US 63-02). In this story the Beagle Boys spray a fluorescent substance on Scrooge. This way they can see where he is going while sneaking away to the castle where he has hidden his money from them. When Scrooge sees his own mirror image, he thinks he is a ghost, even though he cannot remember the manner of his death. Donald sends him to a psychologist, and Scrooge realises that he is alive. The Beagle Boys did not enter the place where Scrooge had hidden his money by night, by the way, because they think the house is haunted. We find another example of Scrooge's strong belief in ghosts in "Return to Pizen Bluff" (US 26-04), where Scrooge is dreaming about ghosts. This leads to the rediscovery of a gold mine he left when he was young.

So, the belief in ghosts is strong among the adults in Duckburg. What is it like with the children? In "The Old Castle's Secret" (OS 189-02) Huey, Dewey and Louie show sound scepticism, but there are stories where they are tainted with the adults' belief. In "The Ghost Sheriff of Last Gasp" (WDC 176-02) they have reason to believe that there really is a ghost. The people in the town moved away because of what the thought to be a ghost. They ere wrong, it is the former sheriff who is walking in the mines beneath the town. He has been doing that for 70 years without getting any older. (Some people believe Gyro has solved the mystery of his not getting any older. They believe that is the reason why the ducks do not seem to age.

The mystery of the Ghost Sheriff in Last Gasp is solved, but Huey, Dewey and Louie have obviously forgotten all about this in "Mystery of the Ghost Town Railroad" (US 56-02) where they meet birds dressed as ghosts. (I suspect that the villains in this story have connections with Magica de Spell as the birds can fly with the ghost costume on.

Traditional spirits (jinns like the ones we find in the Arabian Nights) are almost absent in Duckburg - probably because of the distance to Arabia. In Don Rosa's "The Duck Who Never Was" (D 93574) there really is a spirit that can grant wishes. We have all reason to believe that the spirit of the jar (not the lamp!) is an Arabian import, and as such, it is foreign element in Duckburg. That is probably the reason why it is put in the Duckburg Museum.

As to other spirits, I feel that the belief in demons fits better on the page about

The belief in spirits (especially in ghosts) is, as we have seen, strong in Duckburg. The belief is strongest among the adults, but with time it infects even the young ducks. Some reports may suggest that ghosts really exist in the parallel universe of the ducks.