diff --git a/django/core/management/validation.py b/django/core/management/validation.py
index 4ec421c699b..90a78403fbd 100644
--- a/django/core/management/validation.py
+++ b/django/core/management/validation.py
@@ -309,7 +309,7 @@ def get_validation_errors(outfile, app=None):
             # occurs for symmetrical m2m relations to self). If this is the
             # case, there are no clashes to check for this field, as there are
             # no reverse descriptors for this field.
-            if rel_name is not None:
+            if not f.rel.is_hidden():
                 for r in rel_opts.fields:
                     if r.name == rel_name:
                         e.add(opts, "Accessor for m2m field '%s' clashes with field '%s.%s'. Add a related_name argument to the definition for '%s'." % (f.name, rel_opts.object_name, r.name, f.name))
diff --git a/tests/model_validation/models.py b/tests/model_validation/models.py
index 9a2a5f7cd05..d2e77cbcccd 100644
--- a/tests/model_validation/models.py
+++ b/tests/model_validation/models.py
@@ -25,3 +25,24 @@ class ThingWithIterableChoices(models.Model):
     # Testing choices= Iterable of Iterables
     #   See: https://code.djangoproject.com/ticket/20430
     thing = models.CharField(max_length=100, blank=True, choices=Things())
+
+    class Meta:
+        # Models created as unmanaged as these aren't ever queried
+        managed = False
+
+
+class ManyToManyRel(models.Model):
+    thing1 = models.ManyToManyField(ThingWithIterableChoices, related_name='+')
+    thing2 = models.ManyToManyField(ThingWithIterableChoices, related_name='+')
+
+    class Meta:
+        # Models created as unmanaged as these aren't ever queried
+        managed = False
+
+class FKRel(models.Model):
+    thing1 = models.ForeignKey(ThingWithIterableChoices, related_name='+')
+    thing2 = models.ForeignKey(ThingWithIterableChoices, related_name='+')
+
+    class Meta:
+        # Models created as unmanaged as these aren't ever queried
+        managed = False
diff --git a/tests/model_validation/tests.py b/tests/model_validation/tests.py
index ffd0d894126..494af97f961 100644
--- a/tests/model_validation/tests.py
+++ b/tests/model_validation/tests.py
@@ -10,4 +10,6 @@ class ModelValidationTest(TestCase):
         # Validation Tests:
         #   * choices= Iterable of Iterables
         #       See: https://code.djangoproject.com/ticket/20430
+        #   * related_name='+' doesn't clash with another '+'
+        #       See: https://code.djangoproject.com/ticket/21375
         management.call_command("validate", stdout=six.StringIO())