diff --git a/libp2p/pubsub/gossipsub.py b/libp2p/pubsub/gossipsub.py
index 01576ae7..31ab606f 100644
--- a/libp2p/pubsub/gossipsub.py
+++ b/libp2p/pubsub/gossipsub.py
@@ -204,8 +204,9 @@ class GossipSub(IPubsubRouter):
 
         # Add fanout peers to mesh and notifies them with a GRAFT(topic) control message.
         for peer in fanout_peers:
-            self.mesh[topic].append(peer)
-            await self.emit_graft(topic, peer)
+            if peer not in self.mesh[topic]:
+                self.mesh[topic].append(peer)
+                await self.emit_graft(topic, peer)
 
         if topic_in_fanout:
             del self.fanout[topic]
@@ -281,7 +282,12 @@ class GossipSub(IPubsubRouter):
                     self.mesh[topic]
                 )
 
-                for peer in selected_peers:
+                fanout_peers_not_in_mesh = [
+                    peer
+                    for peer in selected_peers
+                    if peer not in self.mesh[topic]
+                ]
+                for peer in fanout_peers_not_in_mesh:
                     # Add peer to mesh[topic]
                     self.mesh[topic].append(peer)
 
@@ -460,9 +466,11 @@ class GossipSub(IPubsubRouter):
 
         # Add peer to mesh for topic
         if topic in self.mesh:
-            self.mesh[topic].append(from_id_str)
+            if from_id_str not in self.mesh[topic]:
+                self.mesh[topic].append(from_id_str)
         else:
-            self.mesh[topic] = [from_id_str]
+            # Respond with PRUNE if not subscribed to the topic
+            await self.emit_prune(topic, sender_peer_id)
 
     async def handle_prune(self, prune_msg, sender_peer_id):
         topic = prune_msg.topicID
diff --git a/tests/pubsub/test_gossipsub.py b/tests/pubsub/test_gossipsub.py
index a8f58d36..bb47135f 100644
--- a/tests/pubsub/test_gossipsub.py
+++ b/tests/pubsub/test_gossipsub.py
@@ -80,8 +80,8 @@ async def test_leave():
 
     # Create pubsub, gossipsub instances
     _, gossipsubs = create_pubsub_and_gossipsub_instances(libp2p_hosts, \
-                                                                SUPPORTED_PROTOCOLS, \
-                                                                10, 9, 11, 30, 3, 5, 0.5)
+                                                          SUPPORTED_PROTOCOLS, \
+                                                          10, 9, 11, 30, 3, 5, 0.5)
 
     gossipsub = gossipsubs[0]
     topic = "test_leave"
@@ -98,6 +98,110 @@ async def test_leave():
     await cleanup()
 
 
+@pytest.mark.asyncio
+async def test_handle_graft(event_loop, monkeypatch):
+    num_hosts = 2
+    libp2p_hosts = await create_libp2p_hosts(num_hosts)
+
+    # Create pubsub, gossipsub instances
+    _, gossipsubs = create_pubsub_and_gossipsub_instances(libp2p_hosts, \
+                                                          SUPPORTED_PROTOCOLS, \
+                                                          10, 9, 11, 30, 3, 5, 0.5)
+
+    index_alice = 0
+    id_alice = str(libp2p_hosts[index_alice].get_id())
+    index_bob = 1
+    id_bob = str(libp2p_hosts[index_bob].get_id())
+    await connect(libp2p_hosts[index_alice], libp2p_hosts[index_bob])
+
+    # Wait 2 seconds for heartbeat to allow mesh to connect
+    await asyncio.sleep(2)
+
+    topic = "test_handle_graft"
+    # Only lice subscribe to the topic
+    await gossipsubs[index_alice].join(topic)
+
+    # Monkey patch bob's `emit_prune` function so we can
+    # check if it is called in `handle_graft`
+    event_emit_prune = asyncio.Event()
+    async def emit_prune(topic, sender_peer_id):
+        event_emit_prune.set()
+
+    monkeypatch.setattr(gossipsubs[index_bob], 'emit_prune', emit_prune)
+
+    # Check that alice is bob's peer but not his mesh peer
+    assert id_alice in gossipsubs[index_bob].peers_gossipsub
+    assert topic not in gossipsubs[index_bob].mesh
+
+    await gossipsubs[index_alice].emit_graft(topic, id_bob)
+
+    # Check that `emit_prune` is called
+    await asyncio.wait_for(
+        event_emit_prune.wait(),
+        timeout=1,
+        loop=event_loop,
+    )
+    assert event_emit_prune.is_set()
+
+    # Check that bob is alice's peer but not her mesh peer
+    assert topic in gossipsubs[index_alice].mesh
+    assert id_bob not in gossipsubs[index_alice].mesh[topic]
+    assert id_bob in gossipsubs[index_alice].peers_gossipsub
+
+    await gossipsubs[index_bob].emit_graft(topic, id_alice)
+
+    await asyncio.sleep(1)
+
+    # Check that bob is now alice's mesh peer
+    assert id_bob in gossipsubs[index_alice].mesh[topic]
+
+    await cleanup()
+
+
+@pytest.mark.asyncio
+async def test_handle_prune():
+    num_hosts = 2
+    libp2p_hosts = await create_libp2p_hosts(num_hosts)
+
+    # Create pubsub, gossipsub instances
+    pubsubs, gossipsubs = create_pubsub_and_gossipsub_instances(libp2p_hosts, \
+                                                                SUPPORTED_PROTOCOLS, \
+                                                                10, 9, 11, 30, 3, 5, 3)
+
+    index_alice = 0
+    id_alice = str(libp2p_hosts[index_alice].get_id())
+    index_bob = 1
+    id_bob = str(libp2p_hosts[index_bob].get_id())
+
+    topic = "test_handle_prune"
+    for pubsub in pubsubs:
+        await pubsub.subscribe(topic)
+
+    await connect(libp2p_hosts[index_alice], libp2p_hosts[index_bob])
+
+    # Wait 3 seconds for heartbeat to allow mesh to connect
+    await asyncio.sleep(3)
+
+    # Check that they are each other's mesh peer
+    assert id_alice in gossipsubs[index_bob].mesh[topic]
+    assert id_bob in gossipsubs[index_alice].mesh[topic]
+
+    # alice emit prune message to bob, alice should be removed
+    # from bob's mesh peer
+    await gossipsubs[index_alice].emit_prune(topic, id_bob)
+
+    # FIXME: This test currently works because the heartbeat interval
+    # is increased to 3 seconds, so alice won't get add back into
+    # bob's mesh peer during heartbeat.
+    await asyncio.sleep(1)
+
+    # Check that alice is no longer bob's mesh peer
+    assert id_alice not in gossipsubs[index_bob].mesh[topic]
+    assert id_bob in gossipsubs[index_alice].mesh[topic]
+
+    await cleanup()
+
+
 @pytest.mark.asyncio
 async def test_dense():
     # Create libp2p hosts