diff --git a/requirements.txt b/requirements.txt
index 27542e7..5f3da91 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -2,3 +2,4 @@ keyboard==0.13.5
 pyperclip==1.9.0
 pystray==0.19.5
 Pillow==12.0.0
+psutil>=5.8.0
diff --git a/src/main.pyw b/src/main.pyw
index 5f877f3..01a5a7b 100644
--- a/src/main.pyw
+++ b/src/main.pyw
@@ -180,15 +180,26 @@ def cleanup_lock():
 def is_process_running(pid):
     """Check if a process with the given PID is still running."""
     try:
-        os.kill(pid, 0)  # Signal 0 doesn't kill, just checks if process exists
-        return True
-    except OSError:
-        return False
+        import psutil
+        return psutil.pid_exists(pid)
+    except:
+        # Fallback if psutil is not available
+        try:
+            os.kill(pid, 0)  # Signal 0 doesn't kill, just checks if process exists
+            return True
+        except OSError:
+            return False
 
 def ensure_single_instance():
     """Ensure only one instance of the app is running. Exit if another instance exists."""
     if os.path.exists(LOCK_FILE):
         try:
+            # Read the PID from the lock file
+            with open(LOCK_FILE, 'r') as f:
+                old_pid = int(f.read().strip())
+            
+            # Check if the process with that PID is still running
+            if is_process_running(old_pid):
                 # Another instance is running, show message and exit
                 import ctypes
                 ctypes.windll.user32.MessageBoxW(
@@ -198,9 +209,18 @@ def ensure_single_instance():
                     0x40  # MB_ICONINFORMATION
                 )
                 os._exit(1)
+            else:
+                # Process is not running, remove the stale lock file
+                try:
+                    os.remove(LOCK_FILE)
+                except OSError:
+                    pass
         except (ValueError, OSError):
-            # Lock file exists but is invalid or unreadable, clean it up
-            pass
+            # Lock file exists but is invalid or unreadable, clean it up and continue
+            try:
+                os.remove(LOCK_FILE)
+            except OSError:
+                pass
     
     # Create lock file with our PID
     with open(LOCK_FILE, 'w') as f: