A Diplexer is a 3-port passive device that allows two different devices to share a common communication channel. It consists of two filters (Low Pass, High Pass or Band Pass) at different frequencies connected to a single antenna. In the figure below, Signal A at Frequency A enters the Diplexer and passes through Filter A to the antenna. Singal B at frequency B, passes through Filter B to the same antenna. Both the signals need to be at different frequencies by a significant percentage, so that filters can easily sort them.

A Duplexer is a 3-port device that allows the transmitter and receiver to use a single antenna, while operating at the same/similar frequencies. It is a device that allows two-way communication over a single channel by isolating the receiver from transmitter while transmitting a pulse and isolating the transmitter from receiver while receiving a pulse, allowing them to share the same antenna. In a duplexer there is no direct path between the transmitter and receiver. It can be thought of as a circulator i.e the signal from port 1 is routed to port 2 and the signal from port 2 is routed to port 3. Port 1 and Port 3 are isolated from each other.