Perhaps you've seen a few of these old cameras around.
The Twin Lens Reflex or TLR was a camera design pioneered around 1870. The idea was that you could have the shutter-less top lens remain open and project the image onto a viewing screen, which allowed you to frame and focus, while the shutter on the bottom lens would only open to expose the film.
The two lenses were fixed together on the front plate of the camera, so that if you turned the focus knob, the entire front plate of the camera would move, maintaining the same focus on the screen as would be applied to the film.
To be blunt, the Kodak Duaflex was crap. It had a bakelite body, and a cheap f8 lens with fixed focus. The one pictured above belonged to my great grandfather, and while it looks beautiful, the shutter sticks. It's not worth repairing since they usually sell for around $6 on eBay. The camera repair shop has a pile of them that are being turned into clocks, nightlights and other nostalgic pieces. Kodak's business plan was to churn out cheap cameras and make their money on film. The Duaflex is about one step up from a cardboard disposable camera.
The Rolleiflex is perhaps the most famous of the TLRs. Rollei also made the Rolleicord, a more affordable version of the camera, which is currently affordable in the second-hand market (although popularity seems to be growing).
I came across this Rolleicord IV dating from the mid-1950s at Professional Camera Repair when I dropped some of my Leica lenses off yesterday to see if they could be cleaned. Cosmetically, this guy has seen better days, but the lenses were very clear for the age of the camera, and the wonderful part about buying from a camera repair shop rather than eBay or craigslist is that the cameras definitely work.
One advantage of the TLR is its simplicity. Unlike an SLR, you don't need to rely on the mirror flipping up and down. Like a rangefinder you continue to see your subject framed in the viewfinder during the shot, but unlike a rangefinder, no complicated, expensive mechanism is necessary for focus.
At some point my little Rolleicord was transplanted with a Yashica lid and magnifier. It fits and functions fine, so I don't know if it's worth trying to find an original to swap it back. However, I do plan to put in a brighter modern glass focusing screen and to replace the leatherette. I'll document both projects for the blog.
One note, if you're looking for a TLR, always go with one that uses 120 film. 620 film is no longer made, and while you can use 120 film in a 620 camera, it involves re-rolling the film in total darkness onto 620 spools every single time you load the camera. Not fun.
Many of the Rolleis, Yashicas and Ikoflexes actually had factory adapter kits that allowed them to also utilize 35mm film as it became more and more popular. The Rolleikin kit works on most Rolleiflex and Rolleicord cameras.