diff --git a/Lib/site.py b/Lib/site.py
index 52dd9648734c3e..2142495a887b75 100644
--- a/Lib/site.py
+++ b/Lib/site.py
@@ -390,36 +390,36 @@ def addsitedir(sitedir, known_paths=None, *, defer_processing_start_files=False)
     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
+        try:
+            names = os.listdir(sitedir)
+        except OSError:
+            return
 
-    # The following phases are defined by PEP 829.
-    # Phases 1-3: Read .pth files, accumulating paths and import lines.
-    pth_names = sorted(
-        name for name in names
-        if name.endswith(".pth") and not name.startswith(".")
-    )
-    for name in pth_names:
-        _read_pth_file(sitedir, name, known_paths)
-
-    # Phases 6-7: Discover .start files and accumulate their entry points.
-    # Import lines from .pth files with a matching .start file are discarded
-    # at flush time by _exec_imports().
-    start_names = sorted(
-        name for name in names
-        if name.endswith(".start") and not name.startswith(".")
-    )
-    for name in start_names:
-        _read_start_file(sitedir, name)
-
-    # Generally, when addsitedir() is called explicitly, we'll want to process
-    # all the startup file data immediately.  However, when called through
-    # main(), we'll want to batch up all the startup file processing.  main()
-    # will set this flag to True to defer processing.
-    if not defer_processing_start_files:
-        process_startup_files()
+        # The following phases are defined by PEP 829.
+        # Phases 1-3: Read .pth files, accumulating paths and import lines.
+        pth_names = sorted(
+            name for name in names
+            if name.endswith(".pth") and not name.startswith(".")
+        )
+        for name in pth_names:
+            _read_pth_file(sitedir, name, known_paths)
+
+        # Phases 6-7: Discover .start files and accumulate their entry points.
+        # Import lines from .pth files with a matching .start file are discarded
+        # at flush time by _exec_imports().
+        start_names = sorted(
+            name for name in names
+            if name.endswith(".start") and not name.startswith(".")
+        )
+        for name in start_names:
+            _read_start_file(sitedir, name)
+
+        # Generally, when addsitedir() is called explicitly, we'll want to process
+        # all the startup file data immediately.  However, when called through
+        # main(), we'll want to batch up all the startup file processing.  main()
+        # will set this flag to True to defer processing.
+        if not defer_processing_start_files:
+            process_startup_files()
 
     if reset:
         known_paths = None
diff --git a/Lib/test/test_site.py b/Lib/test/test_site.py
index ac69e2cbdbbe54..04e7aff209d453 100644
--- a/Lib/test/test_site.py
+++ b/Lib/test/test_site.py
@@ -118,6 +118,7 @@ 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',
@@ -202,6 +203,16 @@ def test_addsitedir_explicit_flush(self):
             site.process_startup_files()
             self.pth_file_tests(pth_file)
 
+    def test_addsitedir_idempotent(self):
+        pth_file = PthFile()
+        pth_file.cleanup(prep=True)
+
+        with 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)
+
     def test_addsitedir_dotfile(self):
         pth_file = PthFile('.dotfile')
         # Ensure we have a clean slate.
@@ -411,6 +422,7 @@ 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')
 
     @contextlib.contextmanager
     def create(self):
@@ -427,6 +439,13 @@ 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)
@@ -455,6 +474,8 @@ 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..596ca89958c9ed
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-31-16-15-15.gh-issue-75723.BZ4Rsn.rst
@@ -0,0 +1 @@
+Avoid re-executing ``.pth`` files when :func:`site.addsitedir` is called for a known directory.