activitypub.postprocess_as2: fix de-duping image attachments

for https://github.com/snarfed/bridgy-fed/issues/1000#issuecomment-2119106644

activitypub.py

@@ -685,8 +685,10 @@ def postprocess_as2(activity, orig_obj=None, wrap=True):
     imgs = util.get_list(obj_or_activity, 'image')
     if imgs:
         atts = obj_or_activity['attachment']
-        atts.extend((img if isinstance(img, dict) else {'url': img})
-                    for img in imgs if img not in atts)
+        for img in imgs:
+            if isinstance(img, str):
+                img = {'url': img}
+            add(atts, img)
 
     # cc target's author(s), recipients, mentions
     # https://www.w3.org/TR/activitystreams-vocabulary/#audienceTargeting

tests/test_activitypub.py

@@ -1950,15 +1950,33 @@ class ActivityPubUtilsTest(TestCase):
             'image': [{'url': 'http://r/foo'}, {'url': 'http://r/bar'}],
         }))
 
-    def test_postprocess_as2_image_bare_string_url(self):
+    def test_postprocess_as2_object_image_bare_string_url(self):
         self.assert_equals({
             'id': 'http://localhost/r/xyz',
             'attachment': [{'url': 'http://r/foo'}],
-            'image': ['http://r/foo'],
+            'image': 'http://r/foo',
             'to': [as2.PUBLIC_AUDIENCE],
         }, postprocess_as2({
             'id': 'xyz',
-            'image': ['http://r/foo'],
+            'image': 'http://r/foo',
+        }))
+
+    def test_postprocess_as2_create_activity_image_bare_string_url(self):
+        self.assert_equals({
+            'type': 'Create',
+            'object': {
+                'id': 'http://localhost/r/xyz',
+                'attachment': [{'url': 'http://r/foo'}],
+                'image': ['http://r/foo'],
+                'to': [as2.PUBLIC_AUDIENCE],
+            },
+            'to': [as2.PUBLIC_AUDIENCE],
+        }, postprocess_as2({
+            'type': 'Create',
+            'object': {
+                'id': 'xyz',
+                'image': ['http://r/foo'],
+            },
         }))
 
     def test_postprocess_as2_note(self):