Some (Nikon for one) put a single sensor (front) on some, and dual (front and rear) on others D7100 and none on some. I will say that the infrared wireless remote sensor is not all that great. I have aD7100 and when I want to go wireless remote, I use the Vello which is 2.4 gh and has a range of about 100 yards or so. It also has a removable and interchangable connector that plugs into the cameras (different models use different cables but the 2.4 gh sensor can be used on most cameras if you get the apropriate cable. I have one on my D7100 and an aditional remote for my D70s. The sensor supports 2 sec delay release, continuous release and bulb / Manual release (which I use most of the time for timed exposure) The camera can be set to manual or any of the shutter / shooting modes with it. Many Nikon models also can use the wi-fi (cell phone) adapter, which allows your cell phone / tablet to act as a tethered release. With the wi-fi adapter and the free app on your Android or Apple phone, you can see the image on your phone and control many or all of the camera features from the phone. I also have this but have found that the Vello gets used more than the wi-fi as it uses the battery much faster than the Vello. The Vellow is about $40-$70 with other models above for more money and the wi-fi is about $40 and may be built-in to newer model Nikon cameras. I am sure but don't know that Canon has a similar wi-fi and I do know that Vello is available for Canon and other cameras as well as Nikon (and uses the same sensor but just needs the apropriate connection cable). The Vello automatically codes its self to the remote release that you carry and there are about 16,000,000 chanels so you probably aren't going to have that other photographer triggering your stuff like some remote triggers do. I have the DC-N2 which is good enough for my purposes.
http://www.vellogear.com/listing?category=17772