diff --git a/desktop/main.go b/desktop/main.go
index 3f56748..a17bcf6 100644
--- a/desktop/main.go
+++ b/desktop/main.go
@@ -27,10 +27,33 @@ func main() {
 	app := NewApp()
 	tray := newTray(app)
 
+	// One process per PC, enforced by the OS rather than by hoping.
+	//
+	// The window hides to the tray on close, so the ordinary next thing a shop
+	// assistant does is double-click the desktop shortcut again to get it
+	// back. Without this lock that started a SECOND complete copy: a second
+	// tray icon, a second engine supervisor on the same SQLite WAL and the
+	// same port - the "start twice" failure the agent package exists to
+	// prevent, on the one binary that never had the guard. Seen on a Windows
+	// install as a row of Behavision icons in the tray. A second launch now
+	// only brings the existing window to the front, which is what the person
+	// wanted in the first place.
+	var ctxRef context.Context
+	single := &options.SingleInstanceLock{
+		UniqueId: "ai.loyaly.behavision.desktop",
+		OnSecondInstanceLaunch: func(options.SecondInstanceData) {
+			if ctxRef != nil {
+				runtime.Show(ctxRef)
+				runtime.WindowUnminimise(ctxRef)
+			}
+		},
+	}
+
 	err := wails.Run(&options.App{
-		Title:  "Behavision",
-		Width:  1280,
-		Height: 820,
+		SingleInstanceLock: single,
+		Title:              "Behavision",
+		Width:              1280,
+		Height:             820,
 		// Small enough to still be usable on a cramped shop-counter monitor.
 		MinWidth:    1024,
 		MinHeight:   640,
@@ -40,6 +63,7 @@ func main() {
 		// is where they get the window back.
 		HideWindowOnClose: true,
 		OnStartup: func(ctx context.Context) {
+			ctxRef = ctx
 			app.startup(ctx)
 			tray.start(ctx)
 		},