Apple is limiting iPhone Duo purchases to two per customer. Engadget tried adding more than two units to carts on the Apple Stores for the US, UK, Japan and Spain and got the same warning everywhere: "A maximum of 2 iPhone Duo can be purchased per customer. Please adjust the total quantity of iPhone Duo in your order before checking out." The limit applies to every configuration.
MacRumors reported the US limit on October 19. It is not clear when Apple put it in place. Reddit users in the iPhone Duo community reported mixed experiences, with some saying they ordered four units when pre-orders opened on October 16 and others saying they were capped at two. Apple already listed clear two-per-customer limits in China, Singapore and Hong Kong, but the US site does not mention the limit until a cart goes over two.
Demand has outrun supply. Launch-day delivery slots sold out within minutes, and Engadget says all the stores it checked show a shipping estimate of January 22. Some customers report estimates moving earlier. Best Buy still showed a November 10 pick-up and shipping date for the 512GB model when Engadget checked.
9to5Mac says it has seen dozens of iPhone Duos listed on eBay, some priced above $5,000. It notes Apple has capped supply-constrained iPhones before, in China and in the US, and that organized resellers can get around per-account limits with multiple cards, emails and delivery addresses. It also advises trying a walk-in purchase if you live near an Apple store, though not on launch day. Apple has not published a statement on the cap or on any additional anti-scalping checks.