Improve error message when using @pytest.fixture twice (#11670)

* Improve error message when using @pytest.fixture twice

While obvious in hindsight, this error message confused me. I thought my fixture
function was used in a test function twice, since the wording is ambiguous.

Also, the error does not tell me *which* function is the culprit.

Finally, this adds a test, which wasn't done in
cfd16d0dac where this was originally implemented.

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/_pytest/fixtures.py

@@ -1188,7 +1188,7 @@ class FixtureFunctionMarker:
 
         if getattr(function, "_pytestfixturefunction", False):
             raise ValueError(
-                "fixture is being applied more than once to the same function"
+                f"@pytest.fixture is being applied more than once to the same function {function.__name__!r}"
             )
 
         if hasattr(function, "pytestmark"):

testing/python/fixtures.py

@@ -4353,6 +4353,27 @@ def test_call_fixture_function_error():
         assert fix() == 1
 
 
+def test_fixture_double_decorator(pytester: Pytester) -> None:
+    """Check if an error is raised when using @pytest.fixture twice."""
+    pytester.makepyfile(
+        """
+        import pytest
+
+        @pytest.fixture
+        @pytest.fixture
+        def fixt():
+            pass
+        """
+    )
+    result = pytester.runpytest()
+    result.assert_outcomes(errors=1)
+    result.stdout.fnmatch_lines(
+        [
+            "E * ValueError: @pytest.fixture is being applied more than once to the same function 'fixt'"
+        ]
+    )
+
+
 def test_fixture_param_shadowing(pytester: Pytester) -> None:
     """Parametrized arguments would be shadowed if a fixture with the same name also exists (#5036)"""
     pytester.makepyfile(