diff --git a/js/sim/Simulations.js b/js/sim/Simulations.js
index fb0e95b..4eef2ba 100644
--- a/js/sim/Simulations.js
+++ b/js/sim/Simulations.js
@@ -433,9 +433,11 @@ function Sim(config){
 	////////////////////////
 
 	self.STEP = 0;
+	self.STEP_AT_LAST_CALL = -1;
 
 	self.save = function(){
 		self.STEP = 0;
+		self.STEP_AT_LAST_CALL = -1;
 		self.networkConfig = self.getCurrentNetwork();
 	};
 
@@ -444,6 +446,7 @@ function Sim(config){
 	self.reload = function(){
 		var contagionLevel = self.contagion; // hack for sandbox: keep contagion the same
 		self.STEP = 0;
+		self.STEP_AT_LAST_CALL = -1;
 		self._canPlayBonkSound = true;
 		self.init();
 		self.contagion = contagionLevel;
@@ -451,6 +454,11 @@ function Sim(config){
 
 	self.nextStep = function(){
 
+		// STEP++ is time-based; nextStep() is frame-based. This check ensures 
+		// one nextStep() per STEP to avoid duplicate animations and infections.
+		if(self.STEP == self.STEP_AT_LAST_CALL) return;
+		self.STEP_AT_LAST_CALL = self.STEP;
+
 		// SOUND! If anyone can be infected, play Contagion sound.
 		// Otherwise play Bonk sound ONCE
 		var canBeInfected = self.peeps.filter(function(peep){