diff --git a/Lib/site.py b/Lib/site.py
index 30015b3f26b4b3..2ad5d53975529f 100644
--- a/Lib/site.py
+++ b/Lib/site.py
@@ -242,14 +242,14 @@ def addsitedir(sitedir, known_paths=None):
     if not sitedircase in known_paths:
         sys.path.append(sitedir)        # Add path component
         known_paths.add(sitedircase)
-    try:
-        names = os.listdir(sitedir)
-    except OSError:
-        return
-    names = [name for name in names
-             if name.endswith(".pth") and not name.startswith(".")]
-    for name in sorted(names):
-        addpackage(sitedir, name, known_paths)
+        try:
+            names = os.listdir(sitedir)
+        except OSError:
+            return
+        names = [name for name in names
+                 if name.endswith(".pth") and not name.startswith(".")]
+        for name in sorted(names):
+            addpackage(sitedir, name, known_paths)
     if reset:
         known_paths = None
     return known_paths
diff --git a/Lib/test/test_site.py b/Lib/test/test_site.py
index e7dc5e2611c2de..72fec8e1fd7dd4 100644
--- a/Lib/test/test_site.py
+++ b/Lib/test/test_site.py
@@ -116,6 +116,7 @@ def pth_file_tests(self, pth_file):
                       "%s not in sys.modules" % pth_file.imported)
         self.assertIn(site.makepath(pth_file.good_dir_path)[0], sys.path)
         self.assertFalse(os.path.exists(pth_file.bad_dir_path))
+        self.assertFalse(os.path.exists(pth_file.idempotent_fail_path))
 
     def test_addpackage(self):
         # Make sure addpackage() imports if the line starts with 'import',
@@ -199,6 +200,19 @@ def test_addsitedir(self):
         finally:
             pth_file.cleanup()
 
+    def test_addsitedir_idempotent(self):
+        pth_file = PthFile()
+        pth_file.cleanup(prep=True)
+
+        try:
+            pth_file.create()
+            dirs = set()
+            dirs = site.addsitedir(pth_file.base_dir, dirs)
+            dirs = site.addsitedir(pth_file.base_dir, dirs)
+            self.pth_file_tests(pth_file)
+        finally:
+            pth_file.cleanup(prep=True)
+
     def test_addsitedir_dotfile(self):
         pth_file = PthFile('.dotfile')
         pth_file.cleanup(prep=True)
@@ -414,6 +428,7 @@ def __init__(self, filename_base=TESTFN, imported="time",
         self.bad_dirname = bad_dirname
         self.good_dir_path = os.path.join(self.base_dir, self.good_dirname)
         self.bad_dir_path = os.path.join(self.base_dir, self.bad_dirname)
+        self.idempotent_fail_path = os.path.join(self.base_dir, 'idempotent')
 
     def create(self):
         """Create a .pth file with a comment, blank lines, an ``import
@@ -430,6 +445,13 @@ def create(self):
         try:
             print("#import @bad module name", file=FILE)
             print("\n", file=FILE)
+
+            PROG = f'''\
+if {self.imported!r} in sys.modules:
+    open({self.idempotent_fail_path!r}, 'a+').close()
+'''
+            print(f"import sys; exec({PROG!r})", file=FILE)
+
             print("import %s" % self.imported, file=FILE)
             print(self.good_dirname, file=FILE)
             print(self.bad_dirname, file=FILE)
@@ -454,6 +476,8 @@ def cleanup(self, prep=False):
             os.rmdir(self.good_dir_path)
         if os.path.exists(self.bad_dir_path):
             os.rmdir(self.bad_dir_path)
+        if os.path.exists(self.idempotent_fail_path):
+            os.remove(self.idempotent_fail_path)
 
 class ImportSideEffectTests(unittest.TestCase):
     """Test side-effects from importing 'site'."""
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-03-31-16-15-15.gh-issue-75723.BZ4Rsn.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-31-16-15-15.gh-issue-75723.BZ4Rsn.rst
new file mode 100644
index 00000000000000..a14e47435be567
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-31-16-15-15.gh-issue-75723.BZ4Rsn.rst
@@ -0,0 +1 @@
+Idempotent ``.pth`` file exection in :meth:`site.addsitedir`.