A Diplexer splits an input signal into 2 outputs, i.e a tv splitter. Duplex is 2 things going on at the same time, as in duplex data transmission. A good example of this is a telephone where you have an open channel in both directions. A walky talky would be simplex as you can only listen or talk, but not both at the same time. Hope this helps
With a Diplexer you can feed an aerial to 2 different inputs. Conversely you can have 2 aerials to 1 input. They are usually resistive or inductive/resitive, but can be active also.