[8.0.x] Improve error message when using @pytest.fixture twice (#11958)

Co-authored-by: Florian Bruhin <me@the-compiler.org>

testing/python/fixtures.py

@@ -4352,6 +4352,27 @@ 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(