diff --git a/src/main/index.ts b/src/main/index.ts
index e666950..3596ddb 100644
--- a/src/main/index.ts
+++ b/src/main/index.ts
@@ -1423,4 +1423,8 @@ if (app.requestSingleInstanceLock()) {
     }
     if (f) openFromArg(f);
   });
+} else {
+  // Lost the lock: another Folio is already running. Without this quit the
+  // second process falls through to whenReady() and opens a duplicate window.
+  app.quit();
 }