[flutter_tools][plugins] generated registerWith guards against re-registration (#13384)

packages/flutter_tools/lib/src/plugins.dart

@@ -102,10 +102,22 @@ import {{package}}.{{class}};
  */
 public final class GeneratedPluginRegistrant {
   public static void registerWith(PluginRegistry registry) {
+    if (alreadyRegisteredWith(registry)) {
+      return;
+    }
 {{#plugins}}
     {{class}}.registerWith(registry.registrarFor("{{package}}.{{class}}"));
 {{/plugins}}
   }
+
+  private static boolean alreadyRegisteredWith(PluginRegistry registry) {
+    final String key = GeneratedPluginRegistrant.class.getCanonicalName();
+    if (registry.hasPlugin(key)) {
+      return true;
+    }
+    registry.registrarFor(key);
+    return false;
+  }
 }
 ''';