diff --git a/backend/telemetry_core/tests/e2e_tests.rs b/backend/telemetry_core/tests/e2e_tests.rs
index a9e27ca..5248321 100644
--- a/backend/telemetry_core/tests/e2e_tests.rs
+++ b/backend/telemetry_core/tests/e2e_tests.rs
@@ -260,6 +260,70 @@ async fn e2e_feed_add_and_remove_node() {
     server.shutdown().await;
 }
 
+/// If a node is added, a connecting feed should be told about the new chain.
+/// However, sending a duplicate "system.connected" message from the same node
+/// should not count as a new node but rather the second message should be ignored.
+/// If the node is removed, the feed should be told that the chain has gone.
+#[tokio::test]
+async fn e2e_feed_ignore_duplicate_nodes() {
+    // Connect server and add shard
+    let mut server = start_server_debug().await;
+    let shard_id = server.add_shard().await.unwrap();
+
+    // Connect a node to the shard:
+    let connect = || server.get_shard(shard_id).unwrap().connect_node();
+
+    let (mut node_tx, _) = connect().await.expect("can connect to shard");
+
+    let system_connected = json!(
+        {
+            "id":1,
+            "ts":"2021-07-12T10:37:47.714666+01:00",
+            "payload": {
+                "authority":true,
+                "chain":"Local Testnet",
+                "config":"",
+                "genesis_hash": ghash(1),
+                "implementation":"Substrate Node",
+                "msg":"system.connected",
+                "name":"Alice",
+                "network_id":"12D3KooWEyoppNCUx8Yx66oV9fJnriXwCcXwDDUA2kj6vnc6iDEp",
+                "startup_time":"1625565542717",
+                "version":"2.0.0-07a1af348-aarch64-macos"
+            },
+        }
+    );
+    // Send a "system connected" message:
+    node_tx.send_json_text(system_connected.clone()).unwrap();
+    // ...twice!
+    node_tx.send_json_text(system_connected.clone()).unwrap();
+
+    // Wait a little for this message to propagate to the core
+    // (so that our feed connects after the core knows and not before).
+    tokio::time::sleep(Duration::from_millis(500)).await;
+
+    // Connect a feed.
+    let (_feed_tx, mut feed_rx) = server.get_core().connect_feed().await.unwrap();
+
+    let feed_messages = feed_rx.recv_feed_messages().await.unwrap();
+    assert!(feed_messages.contains(&FeedMessage::AddedChain {
+        name: "Local Testnet".to_owned(),
+        genesis_hash: ghash(1),
+        node_count: 1,
+    }));
+
+    // Disconnect the node:
+    node_tx.close().await.unwrap();
+
+    let feed_messages = feed_rx.recv_feed_messages().await.unwrap();
+    assert!(feed_messages.contains(&FeedMessage::RemovedChain {
+        genesis_hash: ghash(1),
+    }));
+
+    // Tidy up:
+    server.shutdown().await;
+}
+
 /// If nodes connect and the chain name changes, feeds will be told about this
 /// and will keep receiving messages about the renamed chain (despite subscribing
 /// to it by name).
diff --git a/backend/telemetry_shard/src/main.rs b/backend/telemetry_shard/src/main.rs
index 45aa104..3b30728 100644
--- a/backend/telemetry_shard/src/main.rs
+++ b/backend/telemetry_shard/src/main.rs
@@ -352,7 +352,11 @@ where
                     }
 
                     // Note of the message ID, allowing telemetry for it.
-                    allowed_message_ids.insert(message_id, Instant::now());
+                    let prev_join_time = allowed_message_ids.insert(message_id, Instant::now());
+                    if prev_join_time.is_some() {
+                        log::info!("Ignoring duplicate new node with ID {message_id} from {real_addr:?}");
+                        continue;
+                    }
 
                     // Tell the aggregator loop about the new node.
                     log::info!("Adding node with message ID {message_id} from {real_addr:?}");