Fix "Plugin already registered" error with symlinks

Fixes https://github.com/pytest-dev/pytest/issues/4174.

changelog/4174.bugfix.rst

@@ -0,0 +1 @@
+Fix "ValueError: Plugin already registered" with conftest plugins via symlink.

src/_pytest/config/__init__.py

@@ -391,7 +391,7 @@ class PytestPluginManager(PluginManager):
             # and allow users to opt into looking into the rootdir parent
             # directories instead of requiring to specify confcutdir
             clist = []
-            for parent in directory.parts():
+            for parent in directory.realpath().parts():
                 if self._confcutdir and self._confcutdir.relto(parent):
                     continue
                 conftestpath = parent.join("conftest.py")

testing/test_conftest.py

@@ -192,8 +192,10 @@ def test_conftest_confcutdir(testdir):
 )
 def test_conftest_symlink(testdir):
     """Ensure that conftest.py is used for resolved symlinks."""
-    realtests = testdir.tmpdir.mkdir("real").mkdir("app").mkdir("tests")
+    real = testdir.tmpdir.mkdir("real")
+    realtests = real.mkdir("app").mkdir("tests")
     testdir.tmpdir.join("symlinktests").mksymlinkto(realtests)
+    testdir.tmpdir.join("symlink").mksymlinkto(real)
     testdir.makepyfile(
         **{
             "real/app/tests/test_foo.py": "def test1(fixture): pass",
@@ -220,6 +222,10 @@ def test_conftest_symlink(testdir):
     )
     assert result.ret == EXIT_OK
 
+    # Should not cause "ValueError: Plugin already registered" (#4174).
+    result = testdir.runpytest("-vs", "symlink")
+    assert result.ret == EXIT_OK
+
     realtests.ensure("__init__.py")
     result = testdir.runpytest("-vs", "symlinktests/test_foo.py::test1")
     result.stdout.fnmatch_lines(