diff --git a/src/core/ReactComponent.js b/src/core/ReactComponent.js
index 42d4cb62c53a5..898df20c5659f 100644
--- a/src/core/ReactComponent.js
+++ b/src/core/ReactComponent.js
@@ -155,9 +155,21 @@ function validatePropertyKey(name) {
  */
 function validateChildKeys(component) {
   if (Array.isArray(component)) {
+    var keys = {};
     for (var i = 0; i < component.length; i++) {
       var child = component[i];
       if (ReactComponent.isValidComponent(child)) {
+        if (child.props.key != null) {
+          if (keys[child.props.key]) {
+            var currentName = ReactCurrentOwner.current.constructor.displayName;
+            console.warn(
+              'Key collision detected for key "' + child.props.key + '". ' +
+              'Check the render method of ' + currentName + '.'
+            );
+          }
+
+          keys[child.props.key] = true;
+        }
         validateExplicitKey(child);
       }
     }
diff --git a/src/core/__tests__/ReactComponent-test.js b/src/core/__tests__/ReactComponent-test.js
index 1899cfb7ccbaf..1035a09d9e63b 100644
--- a/src/core/__tests__/ReactComponent-test.js
+++ b/src/core/__tests__/ReactComponent-test.js
@@ -22,13 +22,23 @@
 var React;
 var ReactTestUtils;
 
+var mocks;
 var reactComponentExpect;
+var warn;
 
 describe('ReactComponent', function() {
   beforeEach(function() {
     React = require('React');
     ReactTestUtils = require('ReactTestUtils');
     reactComponentExpect = require('reactComponentExpect');
+    mocks = require('mocks');
+
+    warn = console.warn;
+    console.warn = mocks.getMockFunction();
+  });
+
+  afterEach(function() {
+    console.warn = warn;
   });
 
   it('should throw on invalid render targets', function() {
@@ -207,4 +217,30 @@ describe('ReactComponent', function() {
     expect(root.refs.switcher.refs.box.refs.boxDiv._mountDepth).toBe(3);
     expect(root.refs.child.refs.span._mountDepth).toBe(6);
   });
+
+  it('should render even with key collisions', function() {
+    var Component = React.createClass({
+      render: function() {
+        var array = [
+          <span key="1"></span>,
+          <span key="2"></span>,
+          <span key="3"></span>,
+          <span key="4"></span>,
+          <span key="1"></span>
+        ];
+
+        return <div>{array}</div>;
+      }
+    });
+
+    var instance = <Component />;
+    ReactTestUtils.renderIntoDocument(instance);
+
+    expect(console.warn.mock.calls.length).toBe(1);
+
+    expect(console.warn.mock.calls[0][0]).toBe(
+      'Key collision detected for key "1". ' +
+      'Check the render method of Component.'
+    );
+  });
 });
diff --git a/src/utils/__tests__/ReactChildren-test.js b/src/utils/__tests__/ReactChildren-test.js
index b256d3fdf5fa0..544f78134c0e4 100644
--- a/src/utils/__tests__/ReactChildren-test.js
+++ b/src/utils/__tests__/ReactChildren-test.js
@@ -319,18 +319,4 @@ describe('ReactChildren', function() {
       ReactChildren.map(instance.props.children, mapFn);
     }).not.toThrow();
   });
-
-  it('should throw if key provided is a dupe with explicit key', function() {
-    var zero = <div key="something"/>;
-    var one = <div key="something" />;
-
-    var mapFn = function() {return null;};
-    var instance = (
-      <div>{zero}{one}</div>
-    );
-
-    expect(function() {
-      ReactChildren.map(instance.props.children, mapFn);
-    }).toThrow();
-  });
 });
diff --git a/src/utils/flattenChildren.js b/src/utils/flattenChildren.js
index 77650b67aa93a..f43f21a55d0fe 100644
--- a/src/utils/flattenChildren.js
+++ b/src/utils/flattenChildren.js
@@ -18,7 +18,6 @@
 
 "use strict";
 
-var invariant = require('invariant');
 var traverseAllChildren = require('traverseAllChildren');
 
 /**
@@ -27,16 +26,8 @@ var traverseAllChildren = require('traverseAllChildren');
  * @param {!string} name String name of key path to child.
  */
 function flattenSingleChildIntoContext(traverseContext, child, name) {
-  // We found a component instance.
-  var result = traverseContext;
-  invariant(
-    !result.hasOwnProperty(name),
-    'flattenChildren(...): Encountered two children with the same key, `%s`. ' +
-    'Children keys must be unique.',
-    name
-  );
   if (child != null) {
-    result[name] = child;
+    traverseContext[name] = child;
   }
 }
 
diff --git a/src/utils/traverseAllChildren.js b/src/utils/traverseAllChildren.js
index 2fa6861f34bf1..e6969e428b5e5 100644
--- a/src/utils/traverseAllChildren.js
+++ b/src/utils/traverseAllChildren.js
@@ -98,13 +98,18 @@ function wrapUserProvidedKey(key) {
 var traverseAllChildrenImpl =
   function(children, nameSoFar, indexSoFar, callback, traverseContext) {
     var subtreeCount = 0;  // Count of children found in the current subtree.
+    var key;
     if (Array.isArray(children)) {
       for (var i = 0; i < children.length; i++) {
         var child = children[i];
+        key = getComponentKey(child, i);
+        if (traverseContext && traverseContext.hasOwnProperty(key)) {
+          key = getComponentKey(null, i);
+        }
         var nextName = (
           nameSoFar +
           (nameSoFar ? SUBSEPARATOR : SEPARATOR) +
-          getComponentKey(child, i)
+          key
         );
         var nextIndex = indexSoFar + subtreeCount;
         subtreeCount += traverseAllChildrenImpl(
@@ -136,7 +141,7 @@ var traverseAllChildrenImpl =
             'traverseAllChildren(...): Encountered an invalid child; DOM ' +
             'elements are not valid children of React components.'
           );
-          for (var key in children) {
+          for (key in children) {
             if (children.hasOwnProperty(key)) {
               subtreeCount += traverseAllChildrenImpl(
                 children[key],