One of the simplest types of duplex repeaters is the crossband or XBand repeater. This video discuses why crossband repeaters are so much cheaper than conventional repeaters, and lists many of the possible applications for XBand repeaters.
Here's the fourth video in my lecture series on FM voice repeaters. I've recently gotten a contract to design and build a cross-band repeater using a RICK and a pair of Kenwood radios, so this lecture series will possibly keep going. Enjoy.