registerCanListener should be idempotent

firmware/controllers/can/can_listener.h

@@ -36,6 +36,10 @@ class CanListener {
 		return m_next;
 	}
 
+	bool hasNext() const {
+		return m_next;
+	}
+
 	// Return true if the provided frame should be accepted for processing by the listener.
 	// Override if you need more complex logic than comparing to a single ID.
 	virtual bool acceptFrame(const CANRxFrame& frame) const {

firmware/controllers/can/can_rx.cpp

@@ -84,7 +84,23 @@ static void printPacket(CanBusIndex busIndex, const CANRxFrame &rx) {
 
 volatile float canMap = 0;
 
-CanListener *canListeners_head = nullptr;
+struct CanListenerTailSentinel : public CanListener {
+	CanListenerTailSentinel()
+		: CanListener(0)
+	{
+	}
+
+	bool acceptFrame(const CANRxFrame&) const override {
+		return false;
+	}
+
+	void decodeFrame(const CANRxFrame&, efitick_t) override {
+		// nothing to do
+	}
+};
+
+static CanListenerTailSentinel tailSentinel;
+CanListener *canListeners_head = &tailSentinel;
 
 void serviceCanSubscribers(const CANRxFrame &frame, efitick_t nowNt) {
 	CanListener *current = canListeners_head;
@@ -95,8 +111,11 @@ void serviceCanSubscribers(const CANRxFrame &frame, efitick_t nowNt) {
 }
 
 void registerCanListener(CanListener& listener) {
-	listener.setNext(canListeners_head);
-	canListeners_head = &listener;
+	// If the listener already has a next, it's already registered
+	if (!listener.hasNext()) {
+		listener.setNext(canListeners_head);
+		canListeners_head = &listener;
+	}
 }
 
 void registerCanSensor(CanSensorBase& sensor) {