a duplexer is a special diplexer :
A diplexer is called a duplexer when it connects a receiver and transmitter to the same antenna. In that case, the antenna is part of the multiplexed communications channel, so only one diplexer is needed. The design requirements for that duplexer are often severe: the receive and transmit frequency bands are often closely spaced and lots of isolation is required.
a diplexer is the generic name for this kind of device.