diff --git a/solitaire_server/web/game.js b/solitaire_server/web/game.js
index dec4e95..7270052 100644
--- a/solitaire_server/web/game.js
+++ b/solitaire_server/web/game.js
@@ -270,6 +270,11 @@ function startGame(seed) {
 
 // ── Timer ────────────────────────────────────────────────────────────────────
 function startTimer() {
+    // Idempotent: never stack a second interval. The visibilitychange handler
+    // and startGame can both call this, and a stray call (e.g. a load-time
+    // visibilitychange while a timer is already running) would otherwise leak
+    // the old interval and make elapsedSecs increment twice per second.
+    if (timerInterval) return;
     timerInterval = setInterval(() => {
         elapsedSecs++;
         updateTimerDisplay();