A duplexer is not the same as a diplexer. A duplexer is a cavity filter that is tuned to notch a certain frequency. They're frequently used in in-band repeater installations.
A diplexer splits or combines signals from separate frequency ranges. Comet, Diamond, etc. make them for either splitting a multi-band antenna to feed separate, monoband radios or for feeding a radio with multiple outputs into one antenna.
Those are pretty common definitions, I think, but not supported by industry.
Duplexer was intended to mean a device allowing duplex operation, i.e., transmitting and receiving with a single antenna. Doesn't matter what that device is, or how widely separated the frequencies are.
Diplexer was intended to mean a device allowing multiple transmitters to share a common antenna, without regard for duplex operation. Doesn't matter what that device is, or how widely separated the frequencies are.
For example, WCBS-AM and WNBC-AM BC radio stations in NYC share a common antenna. They operate at 880 kHz and 660 kHz respectively (in the same band, within 220 kHz of each other). The device used to allow that is called a "diplexer." I'm pretty familiar with that one, since I've worked on it.