Piranhas can and will eat other Piranhas but usually only when the Piranhas are injured then the other non injured Piranhas will see the injured Piranhas as food and eat them.
The Injured Piranhas will also still continue to eat it's original prey even though they've been eaten by other Piranhas.
Also Piranha fish will eat a person as well and if there are around 500 Piranhas near one person they could eat the flesh off of a person within 5 to 10 minutes so never swim with Piranhas or stick your hand in a tank of Piranhas unless you want your flesh eaten off.