only subscribe to a universe once

wiflash_esp32/sacn.cpp

@@ -35,6 +35,11 @@ ESPAsyncE131::ESPAsyncE131() {
 bool ESPAsyncE131::subscribe(uint16_t num, e131_listen_t type) {
   bool success = false;
 
+  if (universes_.count(num)) {
+    // already subscribed
+    return true;
+  }
+
   switch (type) {
     case E131_UNICAST:
       Serial.println("Unicasting");