fixtures: fix pytest_fixture_post_finalizer sometimes called multiple times

Fixes #5848.

Co-authored-by: Ran Benita <ran@unusedvar.com>

Author: Anton3

changelog/5848.bugfix.rst

@@ -0,0 +1 @@
+:hook:`pytest_fixture_post_finalizer` is no longer called extra times for the same fixture teardown in some cases.

src/_pytest/fixtures.py

@@ -1028,6 +1028,11 @@ def addfinalizer(self, finalizer: Callable[[], object]) -> None:
         self._finalizers.append(finalizer)
 
     def finish(self, request: SubRequest) -> None:
+        if self.cached_result is None:
+            # Already finished. It is assumed that finalizers cannot be added in
+            # this state.
+            return
+
         exceptions: list[BaseException] = []
         while self._finalizers:
             fin = self._finalizers.pop()