diff --git a/scrapy/utils/datatypes.py b/scrapy/utils/datatypes.py
index ca6089e0f..1d56811f0 100644
--- a/scrapy/utils/datatypes.py
+++ b/scrapy/utils/datatypes.py
@@ -79,35 +79,45 @@ class CaselessDict(dict):
         return dict.pop(self, self.normkey(key), *args)
 
 
-class CaseInsensitiveDict(collections.UserDict):
+class CaseInsensitiveDict(collections.UserDict,):
     """A dict-like structure that accepts strings or bytes as keys and allows case-insensitive lookups.
 
     It also allows overriding key and value normalization by defining custom `normkey` and `normvalue` methods.
     """
 
+    def __init__(self, *args, **kwargs) -> None:
+        self._keys: dict = {}
+        super().__init__(*args, **kwargs)
+
     def __getitem__(self, key: AnyStr) -> Any:
-        return super().__getitem__(self.normkey(key))
+        normalized_key = self.normkey(key)
+        return super().__getitem__(self._keys[normalized_key.lower()])
 
     def __setitem__(self, key: AnyStr, value: Any) -> None:
-        super().__setitem__(self.normkey(key), self.normvalue(value))
+        normalized_key = self.normkey(key)
+        if normalized_key.lower() in self._keys:
+            del self[self._keys[normalized_key.lower()]]
+        super().__setitem__(normalized_key, self.normvalue(value))
+        self._keys[normalized_key.lower()] = normalized_key
 
     def __delitem__(self, key: AnyStr) -> None:
-        super().__delitem__(self.normkey(key))
+        normalized_key = self.normkey(key)
+        stored_key = self._keys.pop(normalized_key.lower())
+        super().__delitem__(stored_key)
 
     def __contains__(self, key: AnyStr) -> bool:  # type: ignore[override]
-        return super().__contains__(self.normkey(key))
-
-    def normkey(self, key: AnyStr) -> AnyStr:
-        """Method to normalize dictionary key access"""
-        return key.lower()
-
-    def normvalue(self, value: Any) -> Any:
-        """Method to normalize values prior to be set"""
-        return value
+        normalized_key = self.normkey(key)
+        return normalized_key.lower() in self._keys
 
     def __repr__(self) -> str:
         return f"<{self.__class__.__name__}: {super().__repr__()}>"
 
+    def normkey(self, key: AnyStr) -> AnyStr:
+        return key
+
+    def normvalue(self, value: Any) -> Any:
+        return value
+
 
 class LocalCache(collections.OrderedDict):
     """Dictionary with a finite number of keys.
diff --git a/tests/test_utils_datatypes.py b/tests/test_utils_datatypes.py
index c033cd537..5faaabe81 100644
--- a/tests/test_utils_datatypes.py
+++ b/tests/test_utils_datatypes.py
@@ -1,4 +1,5 @@
 import copy
+from typing import Iterator
 import unittest
 import warnings
 from collections.abc import Mapping, MutableMapping
@@ -191,8 +192,16 @@ class CaseInsensitiveDictTest(CaseInsensitiveDictMixin, unittest.TestCase):
     dict_class = CaseInsensitiveDict
 
     def test_repr(self):
-        d = self.dict_class({"foo": "bar"})
-        self.assertEqual(repr(d), "<CaseInsensitiveDict: {'foo': 'bar'}>")
+        d1 = self.dict_class({"foo": "bar"})
+        self.assertEqual(repr(d1), "<CaseInsensitiveDict: {'foo': 'bar'}>")
+        d2 = self.dict_class({"AsDf": "QwErTy", "FoO": "bAr"})
+        self.assertEqual(repr(d2), "<CaseInsensitiveDict: {'AsDf': 'QwErTy', 'FoO': 'bAr'}>")
+
+    def test_iter(self):
+        d = self.dict_class({"AsDf": "QwErTy", "FoO": "bAr"})
+        iterkeys = iter(d)
+        self.assertIsInstance(iterkeys, Iterator)
+        self.assertEqual(list(iterkeys), ["AsDf", "FoO"])
 
 
 class CaselessDictTest(CaseInsensitiveDictMixin, unittest.TestCase):