Apple has acknowledged a problem with the inner screens of its new $1,999 foldable iPhone Duo after customers reported cracks and bright lines appearing near the hinge — in some cases within hours of unboxing the pricey gadget.

The tech giant said Monday that it would replace affected devices — just three days after the highly anticipated phone went on sale Friday, drawing massive crowds and overnight lines outside Apple stores across the country.

“We are aware of an issue impacting the inner display on a small number of iPhone Duo units,” an Apple representative told CNBC. “If users experience issues with their iPhone Duo display, they should contact AppleCare for a replacement.”

An Australian buyer posting as Midnight_Slump on Reddit said his Duo’s screen cracked after roughly six hours of ownership and fewer than 20 openings.

The customer said he felt something beneath the display near the hinge while showing the device to a friend, then heard an audible crack as he continued unfolding it.

After Apple offered an express replacement that required a hold for the full price of the phone, he went to a nearby Apple Store, where employees swapped the damaged handset for a new one on the spot.

“Day zero, six hours of ownership, and already on my second phone,” he wrote.

In Dallas, a Reddit user identified as leftyhyzer16 said his Duo’s hinge became unusually tight after being folded about four times during setup, then made a loud pop when he pushed past the resistance.

The customer said AT&T warned that replacing the phone through the carrier could push him to the back of the order queue in January.

“Drove down to the Dallas Apple Store and they thankfully had one in stock and swapped it out immediately, no charge,” he wrote.

The complaints threaten to cast a shadow over Apple’s entry into the foldable smartphone market, where durability has long been a concern.

“At $1,999, confidence is part of the product,” Ravi Sawhney, founder and CEO of industrial design firm RKS Design, told The Post.

“People are buying the freedom to use something advanced without thinking constantly about how to protect it,” he added.

“The risk for Apple is that opening the phone becomes a moment of hesitation,” Sawhney continued.

“Once people start wondering whether an ordinary interaction could damage their purchase, that changes their relationship with the product.”

Sawhney cautioned against assuming the hinge is responsible for the defects.

“A bright line tells us the display has malfunctioned. It does not, by itself, tell us what caused it,” he said.

“With a folding product, you have to examine the entire mechanical system: the display layers, their support, the hinge movement, the clearances and the assembly tolerances.”

Investigators would need to compare damaged devices with unaffected phones from different production batches and attempt to reproduce the failures under controlled conditions, according to Sawhney.

A recurring problem in properly assembled devices could indicate a design flaw, while defects concentrated in particular batches could point to manufacturing inconsistencies, he said.

Apple has not disclosed the precise cause of the failures or how many devices have been affected.

The company says most units are not susceptible to the problem and that affected phones tend to show symptoms quickly, according to Bloomberg.

The financial stakes are substantial for buyers.

Apple lists the cost of an out-of-warranty inner-screen repair at $999 — roughly half the Duo’s starting price — although that does not mean customers affected by the acknowledged issue will be charged for replacements.

The early troubles follow a frenzied debut that saw shoppers camping outside Apple stores, including one customer who reportedly waited 18 hours outside the Fifth Avenue flagship in Manhattan.

Some locations exhausted their walk-in supplies shortly after opening, while others turned away disappointed customers who had spent hours in line.

Sawhney said Apple’s handling of the complaints could prove critical to maintaining customer trust in the new product.

“If these failures prove isolated and Apple handles them well, confidence can recover,” he said.

“If uncertainty persists, prospective buyers may postpone purchasing — and that hesitation can extend to foldables generally.”