Get a unique filename.

Closes #3447298

src/Plugin/QueueWorker/ImageResize.php

@@ -165,6 +165,8 @@ class ImageResize extends QueueWorkerBase implements ContainerFactoryPluginInter
       if ($config->get('extension') != $current_extension) {
         $image->convert($config->get('extension'));
         $new_uri = str_replace('.' . $current_extension, '.' . $config->get('extension'), $file->getFileUri());
+        // Get a unique filename.
+        $new_uri = $this->fileSystem->getDestinationFilename($new_uri, FileSystemInterface::EXISTS_RENAME);
       }
     }