Relay with arduino/microcontroller, diode needed?
If you're still talking about the relay module from your other question, the answer is no, the diode is not required. This uses solid state relays which don't have a coil. It's that coil in an electromechanical relay that causes a voltage peak when switching off, against which the transistor has to be protected.  

If you would be referring to one of the other modules using electromechanical relays, like this one, those have a protection diode on board.