Checking the file at the destination before downloading.

Closes #3484576

src/Plugin/migrate/process/MWFileCopy.php

@@ -96,6 +96,10 @@ class MWFileCopy extends FileProcessBase implements ContainerFactoryPluginInterf
     }
     [$source, $destination] = $value;
 
+    if (file_exists($destination)) {
+      return $destination;
+    }
+
     // If the source path or URI represents a remote resource, delegate to the
     // download plugin.
     if (!$this->isLocalUri($source)) {