No infinite derivatives.

src/EventGenerator/EmitEvent.php

@@ -157,6 +157,10 @@ abstract class EmitEvent extends ConfigurableActionBase implements ContainerFact
 
       $user = $this->entityTypeManager->getStorage('user')->load($this->account->id());
       $data = $this->generateData($entity);
+      // If $data is the bool false, then abort. No error, but don't emit event.
+      if ($data === FALSE) {
+        return;
+      }
 
       $event = $this->eventDispatcher->dispatch(
         StompHeaderEvent::EVENT_NAME,

src/Plugin/Action/AbstractGenerateDerivative.php

@@ -60,6 +60,13 @@ class AbstractGenerateDerivative extends AbstractGenerateDerivativeBase {
       throw new \RuntimeException("Could not locate taxonomy term with uri: " . $this->configuration['derivative_term_uri'], 500);
     }
 
+    // See if there is a destination media already set, and abort if it's the
+    // same as the source media. Dont cause an error, just don't continue.
+    $derivative_media = $this->utils->getMediaWithTerm($entity, $derivative_term);
+    if (!is_null($derivative_media) && $derivative_media->id() == $source_media->id()) {
+      return FALSE;
+    }
+
     $route_params = [
       'node' => $entity->id(),
       'media_type' => $this->configuration['destination_media_type'],