diff --git a/neutron_lib/api/validators/__init__.py b/neutron_lib/api/validators/__init__.py
index f1856af6f..83bde1e2c 100644
--- a/neutron_lib/api/validators/__init__.py
+++ b/neutron_lib/api/validators/__init__.py
@@ -78,14 +78,14 @@ def _collect_duplicates(data_list):
 
     :param data_list: A list of items to check for duplicates. The list may
         include dict items.
-    :returns: A set of items that are duplicates in data_list. If no
-        duplicates are found, the returned set is empty.
+    :returns: A list of items that are duplicates in data_list. If no
+        duplicates are found, the returned list is empty.
     """
     seen = []
-    dups = set()
+    dups = []
     for datum in data_list:
-        if datum in seen:
-            dups.add(datum)
+        if datum in seen and datum not in dups:
+            dups.append(datum)
             continue
         seen.append(datum)
     return dups
diff --git a/neutron_lib/tests/unit/api/validators/test_validators.py b/neutron_lib/tests/unit/api/validators/test_validators.py
index 5da007edb..0a0d0aba7 100644
--- a/neutron_lib/tests/unit/api/validators/test_validators.py
+++ b/neutron_lib/tests/unit/api/validators/test_validators.py
@@ -971,6 +971,24 @@ class TestAttributeValidation(base.BaseTestCase):
         msg = validators._validate_list_of_items_non_empty(mock.Mock(), items)
         self.assertEqual("List should not be empty", msg)
 
+    def test__validate_list_of_items_collect_duplicates(self):
+        items = ['a', 'b', 'duplicate_1', 'duplicate_2', 'duplicate_1',
+                 'duplicate_2', 'duplicate_2', 'c']
+        msg = validators._validate_list_of_items(mock.Mock(), items)
+        error = ("Duplicate items in the list: '%s'"
+                 % 'duplicate_1, duplicate_2')
+        self.assertEqual(error, msg)
+
+        items = [['a', 'b'], ['c', 'd'], ['a', 'b']]
+        msg = validators._validate_list_of_items(mock.Mock(), items)
+        error = "Duplicate items in the list: '%s'" % ['a', 'b']
+        self.assertEqual(error, msg)
+
+        items = [{'a': 'b'}, {'c': 'd'}, {'a': 'b'}]
+        msg = validators._validate_list_of_items(mock.Mock(), items)
+        error = "Duplicate items in the list: '%s'" % {'a': 'b'}
+        self.assertEqual(error, msg)
+
     def test_validate_dict_type(self):
         for value in (None, True, '1', []):
             self.assertEqual("'%s' is not a dictionary" % value,
diff --git a/releasenotes/notes/fix-api-list-validation-collect-duplicates-f4d45bf5d5abbdff.yaml b/releasenotes/notes/fix-api-list-validation-collect-duplicates-f4d45bf5d5abbdff.yaml
new file mode 100644
index 000000000..b33abede4
--- /dev/null
+++ b/releasenotes/notes/fix-api-list-validation-collect-duplicates-f4d45bf5d5abbdff.yaml
@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed an issue where API validation for duplicate entries in list values
+    would fail with a TypeError in case the values in the list was of type
+    ``dict``.
+    See bug: `1956785 <https://bugs.launchpad.net/neutron/+bug/1956785>`_.
+