The new foldable iPhone Duo is the "thinnest iPhone ever," according to Apple. When unfolded, the device is 5.2mm thick, making it 0.4mm thinner than the 5.6mm iPhone Air . Rumors suggested it could be as thin as 4.5mm when unfolded, similar to Samsung's Galaxy Z Fold8, but it didn't end up that thin. At 5.2mm, it's just a little bit thicker than the 5.1mm…