The leaker Kang says on Weibo that the cracked iPhone Duo inner screens trace back to "defective materials" at one of Apple's two hinge suppliers, Tom's Guide reported. Kang says the hinge design isn't at fault, and that Apple can pick out the affected phones by their serial numbers. Apple still hasn't said what causes the cracks.
When iFixit took a Duo apart last month, it found the breaks cluster just below where the flex cable crosses between the two halves of the phone. Its list of possible causes ran from pressure and a trapped particle to repeated bending and a plain manufacturing defect. Kang's account would be that last one. Several of iFixit's other ideas are about how the hinge is built, and Kang rules those out.
A bad batch would also mean that buyers still waiting for a Duo should get a working phone. Nobody outside Apple knows how many phones are involved, though. Apple says "only a small number" are affected, supply has been tight, and photos on social media can make the problem look bigger than it is. It isn't clear either whether pulling those hinges would stretch delivery times, which already run long.
Apple hasn't announced a way to check your serial number, so owners can't find out ahead of time. For now, Apple is replacing cracked Duos for free under the standard warranty, skipping the usual repair wait, and you don't need AppleCare+. AppleCare is where it told owners to start. Don't wait for a crack if something seems off, either. Owners on social media describe a raised bump on the hinge before the screen goes, and Apple's own support page lists odd noises when folding as a warning sign.