diff --git a/core/java/android/net/ConnectivityManager.java b/core/java/android/net/ConnectivityManager.java
index 0cef280..73f281d 100644
--- a/core/java/android/net/ConnectivityManager.java
+++ b/core/java/android/net/ConnectivityManager.java
@@ -2891,11 +2891,14 @@
         if (callback == null) {
             throw new IllegalArgumentException("null NetworkCallback");
         }
-        if (need == null && action != REQUEST) {
+        if ((need == null) && (action != REQUEST)) {
             throw new IllegalArgumentException("null NetworkCapabilities");
         }
-        // TODO: throw an exception if callback.networkRequest is not null.
-        // http://b/20701525
+        final int targetSdk = mContext.getApplicationInfo().targetSdkVersion;
+        if ((targetSdk > VERSION_CODES.N_MR1) && (callback.networkRequest != null)) {
+            // http://b/20701525
+            throw new IllegalArgumentException("NetworkCallback already registered");
+        }
         final NetworkRequest request;
         try {
             synchronized(sCallbacks) {
diff --git a/tests/net/java/android/net/ConnectivityManagerTest.java b/tests/net/java/android/net/ConnectivityManagerTest.java
index b984bbf..684a101 100644
--- a/tests/net/java/android/net/ConnectivityManagerTest.java
+++ b/tests/net/java/android/net/ConnectivityManagerTest.java
@@ -36,21 +36,36 @@
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+import static org.mockito.Mockito.any;
+import static org.mockito.Mockito.anyInt;
+import static org.mockito.Mockito.when;
 
-import android.net.ConnectivityManager;
-import android.net.NetworkCapabilities;
-
+import android.content.Context;
+import android.content.pm.ApplicationInfo;
+import android.os.Build.VERSION_CODES;
+import android.net.ConnectivityManager.NetworkCallback;
 import android.support.test.filters.SmallTest;
 import android.support.test.runner.AndroidJUnit4;
 
-import org.junit.runner.RunWith;
+import org.junit.Before;
 import org.junit.Test;
-
-
+import org.junit.runner.RunWith;
+import org.mockito.Mock;
+import org.mockito.MockitoAnnotations;
 
 @RunWith(AndroidJUnit4.class)
 @SmallTest
 public class ConnectivityManagerTest {
+
+    @Mock Context mCtx;
+    @Mock IConnectivityManager mService;
+
+    @Before
+    public void setUp() {
+        MockitoAnnotations.initMocks(this);
+    }
+
     static NetworkCapabilities verifyNetworkCapabilities(
             int legacyType, int transportType, int... capabilities) {
         final NetworkCapabilities nc = ConnectivityManager.networkCapabilitiesForType(legacyType);
@@ -173,4 +188,34 @@
         verifyUnrestrictedNetworkCapabilities(
                 ConnectivityManager.TYPE_ETHERNET, TRANSPORT_ETHERNET);
     }
+
+    @Test
+    public void testNoDoubleCallbackRegistration() throws Exception {
+        ConnectivityManager manager = new ConnectivityManager(mCtx, mService);
+        NetworkRequest request = new NetworkRequest.Builder().clearCapabilities().build();
+        NetworkCallback callback = new ConnectivityManager.NetworkCallback();
+        ApplicationInfo info = new ApplicationInfo();
+        info.targetSdkVersion = VERSION_CODES.N_MR1 + 1;
+
+        when(mCtx.getApplicationInfo()).thenReturn(info);
+        when(mService.requestNetwork(any(), any(), anyInt(), any(), anyInt())).thenReturn(request);
+
+        manager.requestNetwork(request, callback);
+
+        // Callback is already registered, reregistration should fail.
+        Class<IllegalArgumentException> wantException = IllegalArgumentException.class;
+        expectThrowable(() -> manager.requestNetwork(request, callback), wantException);
+    }
+
+    static void expectThrowable(Runnable block, Class<? extends Throwable> throwableType) {
+        try {
+            block.run();
+        } catch (Throwable t) {
+            if (t.getClass().equals(throwableType)) {
+                return;
+            }
+            fail("expected exception of type " + throwableType + ", but was " + t.getClass());
+        }
+        fail("expected exception of type " + throwableType);
+    }
 }