British Duke is murdered in elevator. Title is inherited by one of a pair of identical twins, both of whom are suspects

Question

During the COVID lockdowns, so some time between March 2020 and June 2021, I read either a novel or a short story, I think the former, that had the following plot elements:

• A duke is gruesomely, swiftly, and efficiently murdered in a London elevator.
• The elevator is in the block of flats where his younger brother's family lives. This younger brother, who was terminally ill, has died very shortly prior to the duke's murder leaving behind a wife, identical twin sons aged 17 or so, and assorted other children.
• The older twin, being heir presumptive to the dukedom, inherits.
• Both twins fall under suspicion. Someone has seen one of the twins near or in the elevator around the time of the murder, and each twin claims to be the one who was seen.
• A part of the story that completely cracked me up is something like this: when the policeman informs the butler that the duke has been murdered, and asks to speak to the family, the butler, without turning a hair, missing a beat, or breaking a sweat, says (of the older twin), "I shall inform His Grace that you wish to speak to him." That's not quite how it goes, maybe it's just that the policeman asks where the various family members are and the butler says "His Grace is within" or something, but the point is, the late Duke is still lying in a pool of blood in the elevator and the butler has already, entirely correctly, transferred the title to the heir.
• Despite the previous bullet point, the aforementioned butler with the imperturbable sang-froid is not the murderer. (I don't remember who the murderer was, but if "the butler did it," I'd've remembered because it's such a cliché.)
• I think one of the younger siblings of the twins grows up to become a policeman because he has enjoyed this investigation.

So much for plot elements. As for non-plot elements:

• I read this on an iPad, so I got it from some library service like Hoopla or similar, or maybe from archive.org. I did not have a physical copy. I don't remember the cover art, and have no idea what the physical dimensions of the book would have been.
• The book was still in copyright. I did not get it from Project Gutenberg or anything like that.
• I don't remember the decade of the book's setting, nor when the book was published except that both of those were 20th century rather than 21st.
• The book is not by Agatha Christie, Josephine Tey, Dorothy L. Sayers, or Margery Allingham.
• It is possibly by Ngaio Marsh, less likely but still possible is Nicholas Blake / Cecil Day-Lewis.

Answer 1

This sounds very much like Ngaio Marsh's book Surfeit of Lampreys, titled Death of a Peer in the United States.

From Wikipedia:

Lord Wutherwood storms out of the Lampreys' top-floor flat and into the lift. He screams for his wife, Violet, to join him and one of the Lamprey twins accompanies Aunt Violet to the lift. Moments later, Violet begins screaming. Gabriel has been skewered through the eyeball and dies a short while later.

And

The Lampreys argue about the twins, Colin and Stephen, because both twins admit to being the one to accompany Aunt Violet to the lift.