From 26f8929f16c514bd9967cd78d8dcd7760b82cc92 Mon Sep 17 00:00:00 2001
From: afenoum <anja1catus@gmail.com>
Date: Sun, 15 Mar 2026 14:18:59 +0300
Subject: Fixed #36913 -- Optimized MultipleChoiceField.validate().

---
 AUTHORS                                                 |  1 +
 django/forms/fields.py                                  |  3 ++-
 .../forms_tests/field_tests/test_multiplechoicefield.py | 17 +++++++++++++++++
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/AUTHORS b/AUTHORS
index 9379176a4c..bea9d3fd4d 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -35,6 +35,7 @@ answer newbie questions, and generally made Django that much better:
     Ahmad Al-Ibrahim
     Ahmed Eltawela <https://github.com/ahmedabt>
     Ahmed Nassar <https://ahmednassar7.github.io/>
+    Aina Anjary Fenomamy R. <anja1catus@gmail.com>
     ajs <adi@sieker.info>
     Akash Agrawal <akashrocksha@gmail.com>
     Akash Kumar Sen <akashkumarsen4@gmail.com>
diff --git a/django/forms/fields.py b/django/forms/fields.py
index 6408be2f06..7f978aa03d 100644
--- a/django/forms/fields.py
+++ b/django/forms/fields.py
@@ -965,7 +965,8 @@ class MultipleChoiceField(ChoiceField):
         if self.required and not value:
             raise ValidationError(self.error_messages["required"], code="required")
         # Validate that each value in the value list is in self.choices.
-        for val in value:
+        # Use set() to avoid redundant validation.
+        for val in set(value):
             if not self.valid_value(val):
                 raise ValidationError(
                     self.error_messages["invalid_choice"],
diff --git a/tests/forms_tests/field_tests/test_multiplechoicefield.py b/tests/forms_tests/field_tests/test_multiplechoicefield.py
index f316e1de8e..90a47ceee0 100644
--- a/tests/forms_tests/field_tests/test_multiplechoicefield.py
+++ b/tests/forms_tests/field_tests/test_multiplechoicefield.py
@@ -76,3 +76,20 @@ class MultipleChoiceFieldTest(SimpleTestCase):
     def test_disabled_has_changed(self):
         f = MultipleChoiceField(choices=[("1", "One"), ("2", "Two")], disabled=True)
         self.assertIs(f.has_changed("x", "y"), False)
+
+    def test_validate_duplicated_values(self):
+        f = MultipleChoiceField(
+            choices=[
+                ("1", "One"),
+                ("2", "Two"),
+                ("3", "Three"),
+                ("4", "Four"),
+                ("5", "Five"),
+            ]
+        )
+        self.assertIsNone(f.validate(["4", "4", "5", "5"]))
+
+    def test_validate_duplicated_invalid_values(self):
+        f = MultipleChoiceField(choices=[("1", "one"), ("2", "Two")])
+        with self.assertRaises(ValidationError):
+            f.validate(["1", "1", "invalid", "invalid"])
-- 
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