Duality

Everything in this movie happens twice.

Twins act in this film even though there are no twin characters (apart from the flashbacks).
Roughly halfway into the movie, we see Holland behind a window and the frame freezes. At the end of the movie, we see Niles behind a window and the frame freezes.
We get two flashbacks to a deadly fall.
First we see a fetus in a water container (the so-called "hydrocephalic baby") and later we see a baby in a wine cask.
Cousin Piggy jumps twice.
Mrs. Rowe has two encounters with rats.
The entire first half of the movie is set in daylight (apart from one scene set at dusk). The second half on the other hand is a journey into darkness (in more than one respect).
Winnie stops Niles' way to the oven twice.
Two grown-ups jump/fall to their death inside the apple cellar.
Holland uses his slingshot twice.
Two characters ask Niles to remember.

A few dialogue repetitions.
Niles and Mrs. Rowe: "Let me go. Let me go." - "No sass, boy. No sass."
Niles whispers "Father" twice.
Niles yells "Mother" twice.
Piggy says "I'm gonna tell" twice.
Ada and Niles: "Now." - "Now."
Niles: "I'm coming. I'm coming."
Holland says the following lines of dialogue twice: "That damn Uncle George", "You're good at the game" and "Get it back".

Niles and Alexandra have two scenes on the staircase outside the house.
Two scenes in the church.
Two scenes in Ada's room.
Two scenes in Alexandra's room.
Holland tries to kill an animal, Niles succeeds in doing so.
Two older women die in close vicinity to Niles.
The two main characters are 11 years old, a repdigit.
An alliteration: Big Bertha.
Twins with two different birthdays (which consist of two digits beginning with 2).
Niles has two identities.
Which of course is an extension of the great game where you also have two identities: your own and the one you assume.
Niles has a very characteristic mirror image.
Two elven-year-olds die.
Niles uses the handsaw twice.
Niles uses the hedge clippers twice.
There's another character with two identities: Bo-Jo, the dog-faced boy.
Two pitchforks play a significant part during the introduction of the great game. Or is it the same one used for two entirely different purposes?
Two birds, a real raven and a weathervane in the shape of a falcon, play a significant part during the introduction of the great game.
Set in two different years. The main plot is set in 1935, the first flashback is set in 1934.
Mysterious Niles makes Mrs. Rowe and his mother guess wrongly. Unsurprisingly, both women are doomed.
Matching clothes for the twins.
Mrs. Rowe uses her rug beater for two different purposes: beating her carpets and beating Niles.
The very first shot introduces the forest with a pan and then zooms in on Niles. About halfway into the movie, a very similar shot is used for the very same location.
Two magic tricks are performed, one by Chan-Yu and one by Niles.

Niles adopts everything (sometimes via the devious route of Holland). A few examples include:
Russell's nickname, Piggy Lookadoo, comes from a book.
So does the story of the changeling.
The intended imitation of Chan-Yu's magic trick will save Niles' life.
"Yowsa" is first said by Uncle George.
The kidnapping of the baby is, as one character puts it, "the Lindbergh thing all over again".

The trap door to the apple cellar slams twice.
At first there's a baby in the cradle. Later it's a changeling (metaphorically speaking).
Mother and daughter both jump/fall down the stairs.
Piggy Lookadoo dies in the story and in real life.
Robert Mulligan attends two tasks: producer and director.
Tom Tryon (an alliteration) also attends two tasks: executive producer and screenwriter.
Winnie is played by an alliteration: Loretta Leversee.
And there's another alliteration in this film: Anthony Adverse.
It is often said that the twins never appear in the same frame together. Well, their two hands do appear together during the ring removal.
Speaking of the ring removal, Niles goes through Holland's death and its aftermath for the second time.
The flashback scenes show the passing of the two most important male characters in the life of Niles: his father and his brother.
The first and final shot of the movie both approach the main character.
Niles, the two-faced boy, is startled by the two-faced man.
Holland annoyed: "Ada says, Ada says." Twice.
Niles reads Grimm's Fairy Tales, written by two brothers.
And now guess how many elves are seen stealing the baby on the picture inside the book.
Niles has an affinity for two birds: a raven (during the introduction of the great game) and a peregrine falcon (the family crest).
The raven serves a dual purpose: as Niles' embodiment of freedom and as the harbinger of Piggy's death. Those things seem to go hand in hand, don't they?
The name of the actor portraying Cousin Piggy is Clarence Crow (another alliteration by the way). So the raven announces the downfall of the crow.
Holland's birthday (March 20th) is also his death-day.
Two 0s in the film's runtime: 100 minutes.
May 23rd of 1972 is of some importance here. For two reasons of course: 1. The film premiered. 2. Coincidentally, it was the 11th birthday of the Udvarnokys (who were born under the astrological sign of Gemini).

Additions?