diff --git a/Pseudonymisation_Gui_V6.py b/Pseudonymisation_Gui_V6.py
index 7ed8c5c..27e299c 100644
--- a/Pseudonymisation_Gui_V6.py
+++ b/Pseudonymisation_Gui_V6.py
@@ -62,9 +62,24 @@ def main(argv=None) -> int:
     setup_file_logging()
 
     from gui_v6.app import AnonymisationApp
+    from gui_v6.single_instance import AlreadyRunningError, SingleInstance
 
-    application = AnonymisationApp()
-    application.mainloop()
+    guard = SingleInstance()
+    try:
+        guard.acquire()
+    except AlreadyRunningError:
+        try:
+            import tkinter.messagebox as mb
+
+            mb.showinfo("Anonymisation", "L'application est déjà ouverte.")
+        except Exception:
+            print("L'application est déjà ouverte.")
+        return 0
+    try:
+        application = AnonymisationApp()
+        application.mainloop()
+    finally:
+        guard.release()
     return 0
 
 
diff --git a/gui_v6/single_instance.py b/gui_v6/single_instance.py
new file mode 100644
index 0000000..b3b4c53
--- /dev/null
+++ b/gui_v6/single_instance.py
@@ -0,0 +1,70 @@
+"""Protection multi-instance de la GUI V6 (P0-7).
+
+- Windows (frozen) : mutex nommé kernel via ctypes — c'est CE nom que l'installeur
+  Inno détecte (``AppMutex``) pour fermer l'app avant une mise à jour (D8).
+- POSIX (dev/test) : verrou ``fcntl`` exclusif sur un fichier dans le dossier app.
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+from pathlib import Path
+
+# Nom partagé avec installer/Anonymisation.iss (AppMutex). NE PAS modifier seul.
+APP_MUTEX_NAME = "AivanonymAnonymisationV6"
+
+
+class AlreadyRunningError(RuntimeError):
+    """Une autre instance de l'application est déjà en cours d'exécution."""
+
+
+def _lock_dir() -> Path:
+    base = os.environ.get("LOCALAPPDATA")
+    root = Path(base) if base else Path.home() / ".local" / "share"
+    d = root / "Aivanov" / "Anonymisation"
+    d.mkdir(parents=True, exist_ok=True)
+    return d
+
+
+class SingleInstance:
+    def __init__(self) -> None:
+        self._handle = None  # mutex Windows
+        self._fh = None       # file handle POSIX
+
+    def acquire(self) -> None:
+        if sys.platform.startswith("win"):
+            self._acquire_windows()
+        else:
+            self._acquire_posix()
+
+    def _acquire_windows(self) -> None:  # pragma: no cover (exécuté sur Windows)
+        import ctypes
+
+        ERROR_ALREADY_EXISTS = 183
+        handle = ctypes.windll.kernel32.CreateMutexW(None, False, APP_MUTEX_NAME)
+        if not handle or ctypes.windll.kernel32.GetLastError() == ERROR_ALREADY_EXISTS:
+            raise AlreadyRunningError("L'application est déjà ouverte.")
+        self._handle = handle
+
+    def _acquire_posix(self) -> None:
+        import fcntl
+
+        path = _lock_dir() / "instance.lock"
+        fh = open(path, "w")
+        try:
+            fcntl.flock(fh.fileno(), fcntl.LOCK_EX | fcntl.LOCK_NB)
+        except OSError:
+            fh.close()
+            raise AlreadyRunningError("L'application est déjà ouverte.")
+        self._fh = fh
+
+    def release(self) -> None:
+        if self._handle is not None:  # pragma: no cover
+            import ctypes
+
+            ctypes.windll.kernel32.CloseHandle(self._handle)
+            self._handle = None
+        if self._fh is not None:
+            self._fh.close()
+            self._fh = None
diff --git a/tests/unit/test_gui_v6_single_instance.py b/tests/unit/test_gui_v6_single_instance.py
new file mode 100644
index 0000000..6497750
--- /dev/null
+++ b/tests/unit/test_gui_v6_single_instance.py
@@ -0,0 +1,33 @@
+import pytest
+
+from gui_v6.single_instance import (
+    APP_MUTEX_NAME,
+    AlreadyRunningError,
+    SingleInstance,
+)
+
+
+def test_mutex_name_is_stable():
+    # Nom partagé avec l'installeur (Inno AppMutex). Ne pas changer sans MAJ .iss.
+    assert APP_MUTEX_NAME == "AivanonymAnonymisationV6"
+
+
+def test_second_instance_is_rejected(tmp_path, monkeypatch):
+    monkeypatch.setenv("LOCALAPPDATA", str(tmp_path))
+    first = SingleInstance()
+    first.acquire()
+    try:
+        with pytest.raises(AlreadyRunningError):
+            SingleInstance().acquire()
+    finally:
+        first.release()
+
+
+def test_release_allows_reacquire(tmp_path, monkeypatch):
+    monkeypatch.setenv("LOCALAPPDATA", str(tmp_path))
+    a = SingleInstance()
+    a.acquire()
+    a.release()
+    b = SingleInstance()
+    b.acquire()  # ne lève pas
+    b.release()