Intriguingly, most reports on this feature have said it will be exclusive to the iPhone 7 Plus, which means that users will have to pay more upfront to get it. MacRumors points to a new research note from KGI Securities analyst Ming-Chi Kuo that comes the closest anybody has seen to confirming this rumor is true.
"New iPhone shipments to be capped by similar form factor as iPhone 6s & 6s Plus," he writes. "Top hardware upgrade is dual-camera (5.5-inch model only), though many competing models with dual-camera will launch soon, joining others already on the market; first impressions could underwhelm."
Ming-Chi Kuo has been remarkably accurate in the past when describing details about new iPhone features, so if he's writing this in his note there's a good bet that Apple is at least strongly considering making the dual camera exclusive to the iPhone 7 Plus.
It's also interesting to see that he thinks the first impressions of the device could "underwhelm," which suggests that we shouldn't expect a major leap forward like the one we saw going from the iPhone 5s to the iPhone 6. That said, there's still a lot of time left before Apple unveils the device and a lot could change between now and September.