diff --git a/JSONObject.java b/JSONObject.java
index a55ecf8..1b7b0a1 100644
--- a/JSONObject.java
+++ b/JSONObject.java
@@ -237,8 +237,7 @@ public class JSONObject {
             if (key != null) {
                 // Check if key exists
                 if (this.opt(key) != null) {
-                    // back one token to point to the last key character
-                    x.back();
+                    // key already exists
                     throw x.syntaxError("Duplicate key \"" + key + "\"");
                 }
                 // Only add value if non-null