Fixed #27651 -- Allowed M2M to concrete and proxy through model.

django/db/models/fields/related.py

@@ -1447,8 +1447,10 @@ class ManyToManyField(RelatedField):
             if model != self.remote_field.through and model._meta.managed
         }
         m2m_db_table = self.m2m_db_table()
-        if m2m_db_table in registered_tables:
-            model = registered_tables[m2m_db_table]
+        model = registered_tables.get(m2m_db_table)
+        # The second condition allows multiple m2m relations on a model if
+        # some point to a through model that proxies another through model.
+        if model and model._meta.concrete_model != self.remote_field.through._meta.concrete_model:
             if model._meta.auto_created:
                 def _get_field_name(model):
                     for field in model._meta.auto_created._meta.many_to_many:

tests/invalid_models_tests/test_models.py

@@ -899,6 +899,24 @@ class OtherModelTests(SimpleTestCase):
         self.assertEqual(C1.check(), [])
         self.assertEqual(C2.check(), [])
 
+    def test_m2m_to_concrete_and_proxy_allowed(self):
+        class A(models.Model):
+            pass
+
+        class Through(models.Model):
+            a = models.ForeignKey('A', models.CASCADE)
+            c = models.ForeignKey('C', models.CASCADE)
+
+        class ThroughProxy(Through):
+            class Meta:
+                proxy = True
+
+        class C(models.Model):
+            mm_a = models.ManyToManyField(A, through=Through)
+            mm_aproxy = models.ManyToManyField(A, through=ThroughProxy, related_name='proxied_m2m')
+
+        self.assertEqual(C.check(), [])
+
     @isolate_apps('django.contrib.auth', kwarg_name='apps')
     def test_lazy_reference_checks(self, apps):
         class DummyModel(models.Model):