fix: MultipleChoiceField use ordered sort

(cherry picked from commit 8436483)

Author: fbozhang

rest_framework/fields.py

@@ -1469,17 +1469,17 @@ def to_internal_value(self, data):
         if not self.allow_empty and len(data) == 0:
             self.fail('empty')
 
-        return {
-            # Arguments for super() are needed because of scoping inside
-            # comprehensions.
+        # Arguments for super() are needed because of scoping inside
+        # comprehensions.
+        return list(dict.fromkeys([
             super(MultipleChoiceField, self).to_internal_value(item)
             for item in data
-        }
+        ]))
 
     def to_representation(self, value):
-        return {
+        return list(dict.fromkeys([
             self.choice_strings_to_values.get(str(item), item) for item in value
-        }
+        ]))
 
 
 class FilePathField(ChoiceField):