A Duplex stream can be thought of a readable stream with a writable stream. Both are independent and each have separate internal buffer. The reads and writes events happen independently.
Duplex Stream ------------------| Read <----- External Source You ------------------| Write -----> External Sink ------------------| You don't get what you write. It is sent to another source.
A Transform stream is a duplex where the read and write takes place in a causal way. The end points of the duplex stream are linked via some transform. Read requires write to have occurred.
Transform Stream --------------|-------------- You Write ----> ----> Read You --------------|-------------- You write something, it is transformed, then you read something.