From the back of a retail box, we know that the Galaxy S9 duo will bring back the variable aperture the camera would be able to switch between f/1.5 and f/2.4. Behind that aperture will be a 12MP Dual Pixel image sensor and now we have a name.
@MMDDJ_ claims that the Sony IMX345 will be used on the upcoming flagship. Measuring 1/2.55, it will be essentially the same size as the one in the Galaxy S8 and Note8. However information that leaked on Samsung.com suggests the new sensor will support much faster readout - fast enough for 480fps video.
The IMX345 name was reported earlier by Ice Universe (among others), but Sony is yet to officially unveil such a sensor so we cant confirm these details.