diff --git a/django/forms/widgets.py b/django/forms/widgets.py
index ee1966b76f..482c718118 100644
--- a/django/forms/widgets.py
+++ b/django/forms/widgets.py
@@ -75,7 +75,9 @@ class Media(StrAndUnicode):
 
     def add_js(self, data):
         if data:
-            self._js.extend([path for path in data if path not in self._js])
+            for path in data:
+                if path not in self._js:
+                    self._js.append(path)
 
     def add_css(self, data):
         if data:
diff --git a/tests/regressiontests/forms/media.py b/tests/regressiontests/forms/media.py
index fc1b412bcf..f47875db34 100644
--- a/tests/regressiontests/forms/media.py
+++ b/tests/regressiontests/forms/media.py
@@ -112,6 +112,18 @@ media_tests = r"""
 <script type="text/javascript" src="http://media.other.com/path/to/js2"></script>
 <script type="text/javascript" src="https://secure.other.com/path/to/js3"></script>
 
+# Regression check for #12879: specifying the same JS file multiple
+# times in a single Media instance should result in that file only
+# being included once.
+>>> class MyWidget4(TextInput):
+...     class Media:
+...         js = ('/path/to/js1', '/path/to/js1')
+
+>>> w4 = MyWidget4()
+>>> print w4.media
+<script type="text/javascript" src="/path/to/js1"></script>
+
+
 ###############################################################
 # Property-based media definitions
 ###############################################################