Here is a picture of my setup.
The remote flash fires correctly but I don't know why Master flash is firing too? or maybe is it just pre-flash? But it looks pretty bright to me, not sure if it is pre-flash.
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You are using TTL, which means all flashes have to communicate with the camera and there has to be a preflash to determine the flash output.
The process is something like this:
The master has to fire whenever the slaves should fire, which is either to tell them to do the preflash or how to do the actual flash.