diff --git a/scrapy/utils/datatypes.py b/scrapy/utils/datatypes.py
index c020ff4b9..e761a2474 100644
--- a/scrapy/utils/datatypes.py
+++ b/scrapy/utils/datatypes.py
@@ -132,6 +132,22 @@ class CaseInsensitiveDict(collections.UserDict[str | bytes, Any]):
     def __repr__(self) -> str:
         return f"<{self.__class__.__name__}: {super().__repr__()}>"
 
+    # UserDict.copy() shallow-copies the instance, which would share self._keys
+    # between the copy and the original.
+    def __copy__(self) -> Self:
+        new = self.__class__()
+        new.data = self.data.copy()
+        new._keys = self._keys.copy()
+        return new
+
+    copy = __copy__
+
+    # UserDict.__ior__ updates self.data directly, which would leave self._keys
+    # out of date.
+    def __ior__(self, other: Any) -> Self:  # type: ignore[override,misc]
+        self.update(other)
+        return self
+
     def _normkey(self, key: str | bytes) -> str | bytes:
         return key
 
diff --git a/tests/test_utils_datatypes.py b/tests/test_utils_datatypes.py
index fe1f60c7d..f43d20e69 100644
--- a/tests/test_utils_datatypes.py
+++ b/tests/test_utils_datatypes.py
@@ -204,6 +204,15 @@ class TestCaseInsensitiveDictBase(ABC):
         assert h1.get("header1") == h3.get("header1")
         assert h1.get("header1") == h3.get("HEADER1")
 
+    def test_copy_is_independent(self):
+        h1 = self.dict_class({"header1": "value1", "header2": "value2"})
+        for h2 in (copy.copy(h1), h1.copy()):
+            del h2["header1"]
+            h2["header3"] = "value3"
+            assert "header1" in h1
+            assert "header3" not in h1
+            assert dict(h1) == {"header1": "value1", "header2": "value2"}
+
 
 class TestCaseInsensitiveDict(TestCaseInsensitiveDictBase):
     dict_class = CaseInsensitiveDict  # type: ignore[assignment]
@@ -220,6 +229,28 @@ class TestCaseInsensitiveDict(TestCaseInsensitiveDictBase):
         assert isinstance(iterkeys, Iterator)
         assert list(iterkeys) == ["AsDf", "FoO"]
 
+    def test_copy_keeps_values(self):
+        class MyDict(self.dict_class):
+            def _normvalue(self, value):
+                return value + 1
+
+        d = MyDict({"key": 1})
+        for copied in (copy.copy(d), d.copy()):
+            assert copied["key"] == 2
+
+    def test_ior(self):
+        d = self.dict_class({"header1": "value1"})
+        d |= {"HEADER1": "value2", "header2": "value3"}
+        assert len(d) == 2
+        assert d["HeAdEr1"] == "value2"
+        assert d["HeAdEr2"] == "value3"
+
+    def test_ior_mapping(self):
+        d = self.dict_class({"header1": "value1"})
+        d |= self.dict_class({"HEADER1": "value2"})
+        assert len(d) == 1
+        assert d["HeAdEr1"] == "value2"
+
 
 @pytest.mark.filterwarnings("ignore::scrapy.exceptions.ScrapyDeprecationWarning")
 class TestCaselessDict(TestCaseInsensitiveDictBase):