diff --git a/src/isomorphic/children/flattenChildren.js b/src/isomorphic/children/flattenChildren.js
index fa8ea58d74606..bbed350087bab 100644
--- a/src/isomorphic/children/flattenChildren.js
+++ b/src/isomorphic/children/flattenChildren.js
@@ -38,8 +38,10 @@ function flattenSingleChildIntoContext(
         warning(
           false,
           'flattenChildren(...): Encountered two children with the same key, ' +
-            '`%s`. Child keys must be unique; when two children share a key, only ' +
-            'the first child will be used.%s',
+            '`%s`. Keys should be unique so that components maintain their ' +
+            'identity across updates. Non-unique keys may cause children to ' +
+            'be duplicated and/or omitted — the behavior is ' +
+            'unsupported and could change in a future version.%s',
           unescapeInDev(name),
           ReactComponentTreeHook.getStackAddendumByID(selfDebugID),
         );
diff --git a/src/renderers/__tests__/ReactChildReconciler-test.js b/src/renderers/__tests__/ReactChildReconciler-test.js
index 4ad42d83457ce..13826f03989da 100644
--- a/src/renderers/__tests__/ReactChildReconciler-test.js
+++ b/src/renderers/__tests__/ReactChildReconciler-test.js
@@ -60,7 +60,10 @@ describe('ReactChildReconciler', () => {
 
     expectDev(console.error.calls.count()).toBe(1);
     expectDev(console.error.calls.argsFor(0)[0]).toContain(
-      'Child keys must be unique; when two children share a key, only the first child will be used.',
+      'Keys should be unique so that components maintain their identity ' +
+        'across updates. Non-unique keys may cause children to be ' +
+        'duplicated and/or omitted — the behavior is unsupported and ' +
+        'could change in a future version.',
     );
   });
 
@@ -92,9 +95,11 @@ describe('ReactChildReconciler', () => {
       normalizeCodeLocInfo(console.error.calls.argsFor(0)[0]),
     ).toContain(
       'Encountered two children with the same key, `1`. ' +
-        'Child keys must be unique; when two children share a key, ' +
-        'only the first child will be used.\n' +
-        '    in div (at **)\n' +
+        'Keys should be unique so that components maintain their identity ' +
+        'across updates. Non-unique keys may cause children to be ' +
+        'duplicated and/or omitted — the behavior is unsupported and ' +
+        'could change in a future version.',
+      '    in div (at **)\n' +
         '    in Component (at **)\n' +
         '    in Parent (at **)\n' +
         '    in GrandParent (at **)',
@@ -114,7 +119,10 @@ describe('ReactChildReconciler', () => {
 
     expectDev(console.error.calls.count()).toBe(1);
     expectDev(console.error.calls.argsFor(0)[0]).toContain(
-      'Child keys must be unique; when two children share a key, only the first child will be used.',
+      'Keys should be unique so that components maintain their identity ' +
+        'across updates. Non-unique keys may cause children to be ' +
+        'duplicated and/or omitted — the behavior is unsupported and ' +
+        'could change in a future version.',
     );
   });
 
@@ -146,9 +154,11 @@ describe('ReactChildReconciler', () => {
       normalizeCodeLocInfo(console.error.calls.argsFor(0)[0]),
     ).toContain(
       'Encountered two children with the same key, `1`. ' +
-        'Child keys must be unique; when two children share a key, ' +
-        'only the first child will be used.\n' +
-        '    in div (at **)\n' +
+        'Keys should be unique so that components maintain their identity ' +
+        'across updates. Non-unique keys may cause children to be ' +
+        'duplicated and/or omitted — the behavior is unsupported and ' +
+        'could change in a future version.',
+      '    in div (at **)\n' +
         '    in Component (at **)\n' +
         '    in Parent (at **)\n' +
         '    in GrandParent (at **)',
diff --git a/src/renderers/__tests__/ReactMultiChild-test.js b/src/renderers/__tests__/ReactMultiChild-test.js
index f8262f8b8a167..cdf104ba3b3a0 100644
--- a/src/renderers/__tests__/ReactMultiChild-test.js
+++ b/src/renderers/__tests__/ReactMultiChild-test.js
@@ -189,9 +189,11 @@ describe('ReactMultiChild', () => {
         normalizeCodeLocInfo(console.error.calls.argsFor(0)[0]),
       ).toContain(
         'Encountered two children with the same key, `1`. ' +
-          'Child keys must be unique; when two children share a key, ' +
-          'only the first child will be used.\n' +
-          '    in div (at **)\n' +
+          'Keys should be unique so that components maintain their identity ' +
+          'across updates. Non-unique keys may cause children to be ' +
+          'duplicated and/or omitted — the behavior is unsupported and ' +
+          'could change in a future version.',
+        '    in div (at **)\n' +
           '    in WrapperComponent (at **)\n' +
           '    in div (at **)\n' +
           '    in Parent (at **)',
@@ -254,9 +256,11 @@ describe('ReactMultiChild', () => {
         normalizeCodeLocInfo(console.error.calls.argsFor(0)[0]),
       ).toContain(
         'Encountered two children with the same key, `1`. ' +
-          'Child keys must be unique; when two children share a key, ' +
-          'only the first child will be used.\n' +
-          '    in div (at **)\n' +
+          'Keys should be unique so that components maintain their identity ' +
+          'across updates. Non-unique keys may cause children to be ' +
+          'duplicated and/or omitted — the behavior is unsupported and ' +
+          'could change in a future version.',
+        '    in div (at **)\n' +
           '    in WrapperComponent (at **)\n' +
           '    in div (at **)\n' +
           '    in Parent (at **)',
diff --git a/src/renderers/shared/fiber/ReactChildFiber.js b/src/renderers/shared/fiber/ReactChildFiber.js
index d918261264c9e..f5f46616197a2 100644
--- a/src/renderers/shared/fiber/ReactChildFiber.js
+++ b/src/renderers/shared/fiber/ReactChildFiber.js
@@ -718,9 +718,11 @@ function ChildReconciler(shouldClone, shouldTrackSideEffects) {
           }
           warning(
             false,
-            'Encountered two children with the same key, ' +
-              '`%s`. Child keys must be unique; when two children share a key, ' +
-              'only the first child will be used.%s',
+            'Encountered two children with the same key, `%s`. ' +
+              'Keys should be unique so that components maintain their identity ' +
+              'across updates. Non-unique keys may cause children to be ' +
+              'duplicated and/or omitted — the behavior is unsupported and ' +
+              'could change in a future version.%s',
             key,
             getCurrentFiberStackAddendum(),
           );
diff --git a/src/renderers/shared/stack/reconciler/ReactChildReconciler.js b/src/renderers/shared/stack/reconciler/ReactChildReconciler.js
index c43f8cd201478..b15503d5788ad 100644
--- a/src/renderers/shared/stack/reconciler/ReactChildReconciler.js
+++ b/src/renderers/shared/stack/reconciler/ReactChildReconciler.js
@@ -46,9 +46,12 @@ function instantiateChild(childInstances, child, name, selfDebugID) {
     if (!keyUnique) {
       warning(
         false,
-        'flattenChildren(...): Encountered two children with the same key, ' +
-          '`%s`. Child keys must be unique; when two children share a key, only ' +
-          'the first child will be used.%s',
+        'flattenChildren(...):' +
+          'Encountered two children with the same key, `%s`. ' +
+          'Keys should be unique so that components maintain their identity ' +
+          'across updates. Non-unique keys may cause children to be ' +
+          'duplicated and/or omitted — the behavior is unsupported and ' +
+          'could change in a future version.%s',
         KeyEscapeUtils.unescapeInDev(name),
         ReactComponentTreeHook.getStackAddendumByID(selfDebugID),
       );
diff --git a/src/renderers/shared/stack/reconciler/flattenStackChildren.js b/src/renderers/shared/stack/reconciler/flattenStackChildren.js
index da0f50df913cd..295bc25e3903d 100644
--- a/src/renderers/shared/stack/reconciler/flattenStackChildren.js
+++ b/src/renderers/shared/stack/reconciler/flattenStackChildren.js
@@ -57,8 +57,11 @@ function flattenSingleChildIntoContext(
         warning(
           false,
           'flattenChildren(...): Encountered two children with the same key, ' +
-            '`%s`. Child keys must be unique; when two children share a key, only ' +
-            'the first child will be used.%s',
+            '`%s`. ' +
+            'Keys should be unique so that components maintain their identity ' +
+            'across updates. Non-unique keys may cause children to be ' +
+            'duplicated and/or omitted — the behavior is unsupported and ' +
+            'could change in a future version.%s',
           KeyEscapeUtils.unescapeInDev(name),
           ReactComponentTreeHook.getStackAddendumByID(selfDebugID),
         );