diff --git a/src/HTML5/Parser/Tokenizer.php b/src/HTML5/Parser/Tokenizer.php
index feb9a66..c196d1a 100644
--- a/src/HTML5/Parser/Tokenizer.php
+++ b/src/HTML5/Parser/Tokenizer.php
@@ -507,7 +507,7 @@ protected function attribute(&$attributes)
         $this->scanner->whitespace();
 
         $val = $this->attributeValue();
-        if ($isValidAttribute) {
+        if ($isValidAttribute  && !array_key_exists($name, $attributes)) {
             $attributes[$name] = $val;
         }
 
diff --git a/test/HTML5/Parser/TokenizerTest.php b/test/HTML5/Parser/TokenizerTest.php
index 374896f..86401c7 100644
--- a/test/HTML5/Parser/TokenizerTest.php
+++ b/test/HTML5/Parser/TokenizerTest.php
@@ -705,6 +705,14 @@ public function testTagAttributes()
                 ),
                 false,
             ),
+            // duplicate attributes must be ignored
+            '<foo bar="first" bar="second">' => array(
+                'foo',
+                array(
+                    'bar' => 'first',
+                ),
+                false,
+            ),
         );
         $this->isAllGood('startTag', 2, $good);