Overriding fixtures at the same level now triggers a warning.

changelog/12952.improvement.rst

@@ -0,0 +1 @@
+ Overriding fixtures at the same level is considered unintended behavior, now triggers a warning.

src/_pytest/fixtures.py

@@ -1791,6 +1791,10 @@ def parsefactories(
             holderobj_tp = holderobj
 
         self._holderobjseen.add(holderobj)
+
+        # Collect different implementations of the same fixture to check for duplicates.
+        fixture_name_map: dict[str, list[str]] = {}
+
         for name in dir(holderobj):
             # The attribute can be an arbitrary descriptor, so the attribute
             # access below can raise. safe_getattr() ignores such exceptions.
@@ -1811,6 +1815,9 @@ def parsefactories(
 
                 func = obj._get_wrapped_function()
 
+                fixture_name_map.setdefault(fixture_name, [])
+                fixture_name_map[fixture_name].append(f"{func!r}")
+
                 self._register_fixture(
                     name=fixture_name,
                     nodeid=nodeid,
@@ -1821,6 +1828,35 @@ def parsefactories(
                     autouse=marker.autouse,
                 )
 
+        # Check different implementations of the same fixture (#12952).
+        not_by_plugin = nodeid or getattr(holderobj, "__name__", "") == "conftest"
+
+        # If the fixture from a plugin, Skip check.
+        if not_by_plugin:
+            for fixture_name, func_list in fixture_name_map.items():
+                if len(func_list) > 1:
+                    msg = (
+                        f"Fixture definition conflict: \n"
+                        f"{fixture_name!r} has multiple implementations:"
+                        f"{func_list!r}"
+                    )
+
+                    if isinstance(node_or_obj, nodes.Node):
+                        node_or_obj.warn(PytestWarning(msg))
+                    else:
+                        filename = getattr(node_or_obj, "__file__", None)
+                        lineno = 1
+                        if filename is None:
+                            filename = inspect.getfile(type(node_or_obj))
+                            lineno = inspect.getsourcelines(type(node_or_obj))[1]
+
+                        warnings.warn_explicit(
+                            PytestWarning(msg),
+                            category=None,
+                            filename=filename,
+                            lineno=lineno,
+                        )
+
     def getfixturedefs(
         self, argname: str, node: nodes.Node
     ) -> Sequence[FixtureDef[Any]] | None:

testing/python/fixtures.py

@@ -5009,3 +5009,60 @@ def test_result():
     )
     result = pytester.runpytest()
     assert result.ret == 0
+
+
+@pytest.mark.filterwarnings("default")
+def test_fixture_name_conflict(pytester: Pytester) -> None:
+    """Repetitive coverage at the same level is an unexpected behavior (#12952)."""
+    pytester.makepyfile(
+        """
+        import pytest
+
+        @pytest.fixture(name="cache")
+        def c1():  # Create first, but register later
+            return 1
+
+        @pytest.fixture(name="cache")
+        def c0():  # Create later, but register first
+            return 0
+
+        def test_value(cache):
+            assert cache == 0  # Failed, `cache` from c1
+    """
+    )
+
+    result = pytester.runpytest()
+    result.stdout.fnmatch_lines(["* PytestWarning: Fixture definition conflict:*"])
+
+
+@pytest.mark.filterwarnings("default")
+def test_fixture_name_conflict_with_conftest(pytester: Pytester) -> None:
+    """
+    Related to #12952,
+    pyester is unable to capture warnings and errors from root conftest.
+    So in this tests will cover it.
+    """
+    pytester.makeini("[pytest]")
+    pytester.makeconftest(
+        """
+        import pytest
+
+        @pytest.fixture(name="cache")
+        def c1():  # Create first, but register later
+            return 1
+
+        @pytest.fixture(name="cache")
+        def c0():  # Create later, but register first
+            return 0
+    """
+    )
+
+    pytester.makepyfile(
+        """
+        def test_value(cache):
+            assert cache == 0  # Failed, `cache` from c1
+    """
+    )
+
+    with pytest.warns(pytest.PytestWarning):
+        pytester.runpytest()