diff --git a/django/forms/widgets.py b/django/forms/widgets.py
index 7ea3c7bcbf..1b1c1439cb 100644
--- a/django/forms/widgets.py
+++ b/django/forms/widgets.py
@@ -146,8 +146,14 @@ class Media:
 
     def __add__(self, other):
         combined = Media()
-        combined._css_lists = self._css_lists + other._css_lists
-        combined._js_lists = self._js_lists + other._js_lists
+        combined._css_lists = self._css_lists[:]
+        combined._js_lists = self._js_lists[:]
+        for item in other._css_lists:
+            if item and item not in self._css_lists:
+                combined._css_lists.append(item)
+        for item in other._js_lists:
+            if item and item not in self._js_lists:
+                combined._js_lists.append(item)
         return combined
 
 
diff --git a/tests/forms_tests/tests/test_media.py b/tests/forms_tests/tests/test_media.py
index 7d8f978ecd..26f81a4602 100644
--- a/tests/forms_tests/tests/test_media.py
+++ b/tests/forms_tests/tests/test_media.py
@@ -592,3 +592,57 @@ class FormsMediaTestCase(SimpleTestCase):
         # Media ordering does not matter.
         merged = widget1 + widget4
         self.assertEqual(merged._css, {'screen': ['c.css'], 'all': ['d.css', 'e.css']})
+
+    def test_add_js_deduplication(self):
+        widget1 = Media(js=['a', 'b', 'c'])
+        widget2 = Media(js=['a', 'b'])
+        widget3 = Media(js=['a', 'c', 'b'])
+        merged = widget1 + widget1
+        self.assertEqual(merged._js_lists, [['a', 'b', 'c']])
+        self.assertEqual(merged._js, ['a', 'b', 'c'])
+        merged = widget1 + widget2
+        self.assertEqual(merged._js_lists, [['a', 'b', 'c'], ['a', 'b']])
+        self.assertEqual(merged._js, ['a', 'b', 'c'])
+        # Lists with items in a different order are preserved when added.
+        merged = widget1 + widget3
+        self.assertEqual(merged._js_lists, [['a', 'b', 'c'], ['a', 'c', 'b']])
+        msg = (
+            "Detected duplicate Media files in an opposite order: "
+            "['a', 'b', 'c'], ['a', 'c', 'b']"
+        )
+        with self.assertWarnsMessage(RuntimeWarning, msg):
+            merged._js
+
+    def test_add_css_deduplication(self):
+        widget1 = Media(css={'screen': ['a.css'], 'all': ['b.css']})
+        widget2 = Media(css={'screen': ['c.css']})
+        widget3 = Media(css={'screen': ['a.css'], 'all': ['b.css', 'c.css']})
+        widget4 = Media(css={'screen': ['a.css'], 'all': ['c.css', 'b.css']})
+        merged = widget1 + widget1
+        self.assertEqual(merged._css_lists, [{'screen': ['a.css'], 'all': ['b.css']}])
+        self.assertEqual(merged._css, {'screen': ['a.css'], 'all': ['b.css']})
+        merged = widget1 + widget2
+        self.assertEqual(merged._css_lists, [
+            {'screen': ['a.css'], 'all': ['b.css']},
+            {'screen': ['c.css']},
+        ])
+        self.assertEqual(merged._css, {'screen': ['a.css', 'c.css'], 'all': ['b.css']})
+        merged = widget3 + widget4
+        # Ordering within lists is preserved.
+        self.assertEqual(merged._css_lists, [
+            {'screen': ['a.css'], 'all': ['b.css', 'c.css']},
+            {'screen': ['a.css'], 'all': ['c.css', 'b.css']}
+        ])
+        msg = (
+            "Detected duplicate Media files in an opposite order: "
+            "['b.css', 'c.css'], ['c.css', 'b.css']"
+        )
+        with self.assertWarnsMessage(RuntimeWarning, msg):
+            merged._css
+
+    def test_add_empty(self):
+        media = Media(css={'screen': ['a.css']}, js=['a'])
+        empty_media = Media()
+        merged = media + empty_media
+        self.assertEqual(merged._css_lists, [{'screen': ['a.css']}])
+        self.assertEqual(merged._js_lists, [['a']])