diff --git a/src/org/mozilla/javascript/NativeJSON.java b/src/org/mozilla/javascript/NativeJSON.java
index 10a2dabfa0..d3393f6745 100644
--- a/src/org/mozilla/javascript/NativeJSON.java
+++ b/src/org/mozilla/javascript/NativeJSON.java
@@ -9,8 +9,8 @@
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Iterator;
+import java.util.LinkedHashSet;
 import java.util.LinkedList;
-import java.util.List;
 import java.util.Map;
 import java.util.Stack;
 import org.mozilla.javascript.json.JsonParser;
@@ -198,7 +198,7 @@ private static class StringifyState {
                 String indent,
                 String gap,
                 Callable replacer,
-                List<Object> propertyList) {
+                Object[] propertyList) {
             this.cx = cx;
             this.scope = scope;
 
@@ -212,7 +212,7 @@ private static class StringifyState {
         String indent;
         String gap;
         Callable replacer;
-        List<Object> propertyList;
+        Object[] propertyList;
 
         Context cx;
         Scriptable scope;
@@ -223,22 +223,36 @@ public static Object stringify(
         String indent = "";
         String gap = "";
 
-        List<Object> propertyList = null;
+        Object[] propertyList = null;
         Callable replacerFunction = null;
 
         if (replacer instanceof Callable) {
             replacerFunction = (Callable) replacer;
         } else if (replacer instanceof NativeArray) {
-            propertyList = new LinkedList<Object>();
+            LinkedHashSet<Object> propertySet = new LinkedHashSet<Object>();
             NativeArray replacerArray = (NativeArray) replacer;
             for (int i : replacerArray.getIndexIds()) {
                 Object v = replacerArray.get(i, replacerArray);
-                if (v instanceof String || v instanceof Number) {
-                    propertyList.add(v);
-                } else if (v instanceof NativeString || v instanceof NativeNumber) {
-                    propertyList.add(ScriptRuntime.toString(v));
+                if (v instanceof String) {
+                    propertySet.add(v);
+                } else if (v instanceof Number
+                        || v instanceof NativeString
+                        || v instanceof NativeNumber) {
+                    // TODO: This should also apply to subclasses of NativeString and NativeNumber
+                    // once the class, extends, and super keywords are implemented
+                    propertySet.add(ScriptRuntime.toString(v));
                 }
             }
+            // After items have been converted to strings and duplicates removed, transform to an
+            // array and convert indexed keys back to Integers as required for later processing
+            propertyList = new Object[propertySet.size()];
+            int i = 0;
+            for (Object prop : propertySet) {
+                ScriptRuntime.StringIdOrIndex idOrIndex = ScriptRuntime.toStringIdOrIndex(cx, prop);
+                // This will always be a String or Integer
+                propertyList[i++] =
+                        (idOrIndex.stringId == null) ? idOrIndex.index : idOrIndex.stringId;
+            }
         }
 
         if (space instanceof NativeNumber) {
@@ -374,12 +388,12 @@ private static String jo(Scriptable value, StringifyState state) {
         state.indent = state.indent + state.gap;
         Object[] k = null;
         if (state.propertyList != null) {
-            k = state.propertyList.toArray();
+            k = state.propertyList;
         } else {
             k = value.getIds();
         }
 
-        List<Object> partial = new LinkedList<Object>();
+        Collection<Object> partial = new LinkedList<Object>();
 
         for (Object p : k) {
             Object strP = str(p, value, state);
@@ -420,7 +434,7 @@ private static String ja(NativeArray value, StringifyState state) {
 
         String stepback = state.indent;
         state.indent = state.indent + state.gap;
-        List<Object> partial = new LinkedList<Object>();
+        Collection<Object> partial = new LinkedList<Object>();
 
         long len = value.getLength();
         for (long index = 0; index < len; index++) {