diff --git a/main.js b/main.js
index 86d3d24..7e32490 100644
--- a/main.js
+++ b/main.js
@@ -8,6 +8,41 @@ let pendingFilePath = null;
 let fileWatcher = null;
 let closeTimeout = null;
 
+// ===== Single Instance Lock =====
+const gotTheLock = app.requestSingleInstanceLock();
+
+if (!gotTheLock) {
+  app.quit();
+} else {
+  app.on('second-instance', (event, commandLine, workingDirectory) => {
+    // A second instance was launched (e.g. double-clicking a .md file)
+    // Focus the existing window and open the file in it
+    if (mainWindow) {
+      if (mainWindow.isMinimized()) mainWindow.restore();
+      mainWindow.focus();
+
+      // Extract file path from the second instance's command line args
+      const filePath = getFilePathFromArgs(commandLine);
+      if (filePath) {
+        openFileInTab(filePath);
+      }
+    }
+  });
+}
+
+// Extract a valid file path from command line arguments (skip Electron flags)
+function getFilePathFromArgs(args) {
+  for (let i = 1; i < args.length; i++) {
+    const arg = args[i];
+    if (!arg.startsWith('--') && !arg.startsWith('-')) {
+      try {
+        if (fs.existsSync(arg)) return arg;
+      } catch (e) { /* ignore */ }
+    }
+  }
+  return null;
+}
+
 function createWindow() {
   mainWindow = new BrowserWindow({
     width: 1200,
@@ -252,14 +287,9 @@ ipcMain.handle('window:cancelClose', () => {
   }
 });
 
-// Command line file
+// Command line file (first instance)
 function getCommandLineFile() {
-  const args = process.argv.slice(1);
-  if (args.length > 0 && !args[0].startsWith('--')) {
-    const filePath = args[0];
-    if (fs.existsSync(filePath)) return filePath;
-  }
-  return null;
+  return getFilePathFromArgs(process.argv);
 }
 
 // App lifecycle