Apple allocated roughly 80% of its initial iPhone Duo inventory to its retail stores, with locations holding as few as 50 units each, according to Bloomberg ‘s Mark Gurman. Speaking on the latest episode of his Power On podcast , Gurman, citing people very close to the device’s logistics, said most customers who bought a Duo in store on day one got one by waiting in line . Flagship stores were said to have held up to around 250 units, and Fifth Avenue was stocked unusually heavily because Apple executives attended the launch. The inventory split helps explain why pre-orders slipped to late January within minutes on October 16, while most people who queued on launch day walked away with a new phone…