diff --git a/src/qml/compiler/qv4codegen.cpp b/src/qml/compiler/qv4codegen.cpp
index b5b70dbf45..53cbc3c5a0 100644
--- a/src/qml/compiler/qv4codegen.cpp
+++ b/src/qml/compiler/qv4codegen.cpp
@@ -220,7 +220,15 @@ bool Codegen::ScanFunctions::visit(VariableDeclaration *ast)
         _cg->throwSyntaxError(ast->identifierToken, QStringLiteral("Missing initializer in const declaration"));
         return false;
     }
-    _env->enter(ast->name.toString(), ast->expression ? Environment::VariableDefinition : Environment::VariableDeclaration);
+    QString name = ast->name.toString();
+    const Environment::Member *m = 0;
+    if (_env->memberInfo(name, &m)) {
+        if (m->isConstant || ast->readOnly) {
+            _cg->throwSyntaxError(ast->identifierToken, QStringLiteral("Identifier %1 has already been declared").arg(name));
+            return false;
+        }
+    }
+    _env->enter(ast->name.toString(), ast->expression ? Environment::VariableDefinition : Environment::VariableDeclaration, ast->readOnly);
     return true;
 }
 
@@ -391,7 +399,7 @@ void Codegen::ScanFunctions::enterFunction(Node *ast, const QString &name, Forma
         _env->hasNestedFunctions = true;
         // The identifier of a function expression cannot be referenced from the enclosing environment.
         if (expr)
-            _env->enter(name, Environment::FunctionDefinition, expr);
+            _env->enter(name, Environment::FunctionDefinition, false /* readonly */, expr);
         if (name == QLatin1String("arguments"))
             _env->usesArgumentsObject = Environment::ArgumentsObjectNotUsed;
         wasStrict = _env->isStrict;
@@ -2044,7 +2052,7 @@ int Codegen::defineFunction(const QString &name, AST::Node *ast,
     function->column = loc.startColumn;
 
     if (function->usesArgumentsObject)
-        _env->enter(QStringLiteral("arguments"), Environment::VariableDeclaration);
+        _env->enter(QStringLiteral("arguments"), Environment::VariableDeclaration, false /* readonly */);
 
     // variables in global code are properties of the global context object, not locals as with other functions.
     if (_env->compilationMode == FunctionCode || _env->compilationMode == QmlBinding) {
@@ -2062,7 +2070,7 @@ int Codegen::defineFunction(const QString &name, AST::Node *ast,
                 function->LOCAL(inheritedLocal);
                 unsigned tempIndex = entryBlock->newTemp();
                 Environment::Member member = { Environment::UndefinedMember,
-                                               static_cast<int>(tempIndex), 0 };
+                                               static_cast<int>(tempIndex), 0, false /* readonly */ };
                 _env->members.insert(inheritedLocal, member);
             }
         }
diff --git a/src/qml/compiler/qv4codegen_p.h b/src/qml/compiler/qv4codegen_p.h
index 742ee79648..adfdea8805 100644
--- a/src/qml/compiler/qv4codegen_p.h
+++ b/src/qml/compiler/qv4codegen_p.h
@@ -145,6 +145,7 @@ protected:
             MemberType type;
             int index;
             AST::FunctionExpression *function;
+            bool isConstant : 1;
         };
         typedef QMap<QString, Member> MemberMap;
 
@@ -191,6 +192,18 @@ protected:
             return (*it).index;
         }
 
+        bool memberInfo(const QString &name, const Member **m) const
+        {
+            Q_ASSERT(m);
+            MemberMap::const_iterator it = members.find(name);
+            if (it == members.end()) {
+                *m = 0;
+                return false;
+            }
+            *m = &(*it);
+            return true;
+        }
+
         bool lookupMember(const QString &name, Environment **scope, int *index, int *distance)
         {
             Environment *it = this;
@@ -206,7 +219,7 @@ protected:
             return false;
         }
 
-        void enter(const QString &name, MemberType type, AST::FunctionExpression *function = 0)
+        void enter(const QString &name, MemberType type, bool isConstant, AST::FunctionExpression *function = 0)
         {
             if (! name.isEmpty()) {
                 if (type != FunctionDefinition) {
@@ -220,8 +233,10 @@ protected:
                     m.index = -1;
                     m.type = type;
                     m.function = function;
+                    m.isConstant = isConstant;
                     members.insert(name, m);
                 } else {
+                    Q_ASSERT(isConstant == (*it).isConstant);
                     if ((*it).type <= type) {
                         (*it).type = type;
                         (*it).function = function;
diff --git a/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp b/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
index 1f0248c258..53d0912d1c 100644
--- a/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
+++ b/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
@@ -8208,6 +8208,12 @@ void tst_qqmlecmascript::constkw_data()
            "v + i\n"
         << false
         << QVariant(25);
+    QTest::newRow("const-multiple-scopes-same-var")
+        << "const v = 3\n"
+           "function f() { const v = 1; return v; }\n"
+           "v + f()\n"
+        << false
+        << QVariant(4);
 
     // error cases
     QTest::newRow("const-no-initializer")
@@ -8218,6 +8224,25 @@ void tst_qqmlecmascript::constkw_data()
         << "const v = 1, i\n"
         << true
         << QVariant("SyntaxError: Missing initializer in const declaration");
+    QTest::newRow("const-no-duplicate")
+        << "const v = 1, v = 2\n"
+        << true
+        << QVariant("SyntaxError: Identifier v has already been declared");
+    QTest::newRow("const-no-duplicate-2")
+        << "const v = 1\n"
+           "const v = 2\n"
+        << true
+        << QVariant("SyntaxError: Identifier v has already been declared");
+    QTest::newRow("const-no-duplicate-var")
+        << "const v = 1\n"
+           "var v = 1\n"
+        << true
+        << QVariant("SyntaxError: Identifier v has already been declared");
+    QTest::newRow("var-no-duplicate-const")
+        << "var v = 1\n"
+           "const v = 1\n"
+        << true
+        << QVariant("SyntaxError: Identifier v has already been declared");
 }
 
 void tst_qqmlecmascript::constkw()