Flash messages stack and in cases can result in duplicate messages wh…

…en a message is set in AppController::beforeFilter

then a controller receives a post and has to redirect. The message will be set twice.
If a duplicate => false is set, and the message is already in the array, it will not be added.

src/Controller/Component/FlashComponent.php

@@ -47,7 +47,8 @@ class FlashComponent extends Component
         'key' => 'flash',
         'element' => 'default',
         'params' => [],
-        'clear' => false
+        'clear' => false,
+        'duplicate' => true
     ];
 
     /**
@@ -111,6 +112,14 @@ public function set($message, array $options = [])
             $messages = (array)$this->_session->read('Flash.' . $options['key']);
         }
 
+        if ($options['duplicate'] === false) {
+            foreach ($messages as $existingMessage) {
+                if ($existingMessage['message'] == $message) {
+                    return;
+                }
+            }
+        }
+
         $messages[] = [
             'message' => $message,
             'key' => $options['key'],

tests/TestCase/Controller/Component/FlashComponentTest.php

@@ -108,6 +108,13 @@ public function testSet()
         ];
         $result = $this->Session->read('Flash.foobar');
         $this->assertEquals($expected, $result);
+
+
+        $this->Flash->config('duplicate', false);
+        $this->Flash->set('This test message should appear once only');
+        $this->Flash->set('This test message should appear once only');
+        $result = array_slice($this->Session->read('Flash.flash'), -2);
+        $this->assertNotEquals($result[0], $result[1]);
     }
 
     /**