diff --git a/dkim/tests/test_dkim.py b/dkim/tests/test_dkim.py
index c442a96..e5817a8 100644
--- a/dkim/tests/test_dkim.py
+++ b/dkim/tests/test_dkim.py
@@ -77,10 +77,16 @@ class TestParseTagValue(unittest.TestCase):
             {'foo': 'bar', 'baz': 'foo=bar'},
             parse_tag_value('foo=bar;baz=foo=bar'))
 
-    def test_no_value(self):
+    def test_missing_value_is_an_error(self):
         self.assertRaises(
             InvalidTagValueList,
             parse_tag_value, 'foo=bar;baz')
 
+    def test_duplicate_tag_is_an_error(self):
+        self.assertRaises(
+            InvalidTagValueList,
+            parse_tag_value, 'foo=bar;foo=baz')
+
+
 if __name__ == '__main__':
     unittest.main()
diff --git a/dkim/util.py b/dkim/util.py
index 1b73102..d5d6f1e 100644
--- a/dkim/util.py
+++ b/dkim/util.py
@@ -42,5 +42,7 @@ def parse_tag_value(tag_list):
             key, value = tag_spec.split('=', 1)
         except ValueError:
             raise InvalidTagValueList()
+        if key.strip() in tags:
+            raise InvalidTagValueList()
         tags[key.strip()] = value.strip()
     return tags