Don’t Flip Out But Some iPhone Duo Displays Are Already Cracking
One Redditor, user u/Midnight_Slump, even reported a failure in as few as six hours, which we've shared below. As a silver lining, the defect doesn't seem to be universal. Recent iPhone Duo durability testing was much more rigorous with positive results, versus these alarming reports, which should mean that hopefully most iPhone Duos will not suffer from this issue.

For those that do end up with a defective iPhone Duo, I highly recommend taking it to your nearest Apple Store in person. This is what u/Midnight_Slump did to resolve the issue.
Technically you could also talk to Apple support, but as u/Midnight_Slump noted in his original post, an express replacement still required a card hold for the full cost of the phone. There was also no confirmed ETA on shipment despite it being "express." While I don't think Apple Support is being completely unreasonable there, I do think most people would be better served at a proper Apple Store if this happens to them.
For those of you with a non-defective iPhone Duo, the world is your oyster. Developers are eager to optimize their apps for your new smartphone, and accessory manufacturers already have great peripherals available for iPhone Duo. For those of you still waiting on an iPhone Duo you've already ordered, hopefully these users are just extraordinarily unlucky, and these are corner case issues. We'll need a few weeks before we have a true idea of how many launch iPhone Duos suffer from this defect, but to see failures like this so soon out of the gate certainly is alarming.