Fix "Plugin already registered" error with symlinks

Fixes pytest-dev#4174.

Author: blueyed

src/_pytest/config/__init__.py

@@ -380,6 +380,7 @@ def _getconftestmodules(self, path):
         if self._noconftest:
             return []
 
+        path = path.realpath()
         if path.isfile():
             directory = path.dirpath()
         else:

testing/test_conftest.py

@@ -192,8 +192,10 @@ def pytest_addoption(parser):
 )
 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 fixture():
     )
     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(