Duplicate messages deduplicated via client-generated ID before rendering

Real-time transports retry delivery on network errors. Without deduplication, a message sent before a disconnect and resent after reconnect renders twice in the UI — the user sees duplicate chat lines, duplicate notifications, and potentially double-processed side effects. CWE-694 (Use of Multiple Resources with Duplicate Identifier) captures this integrity failure. The fix is straightforward but frequently omitted in AI-generated WebSocket code that copies the happy path without handling retry semantics.

Assign a client-generated ID to each outbound message and maintain a Set of received IDs on the client. Drop any message whose ID has already been processed.

// Receiving
const seen = new Set<string>();
socket.on('message', (msg: { id: string; content: string }) => {
  if (seen.has(msg.id)) return;
  seen.add(msg.id);
  renderMessage(msg);
});

// Sending
const send = (content: string) => {
  const id = nanoid(); // generated on the client
  socket.emit('send_message', { id, content });
};

Bound the seen Set to a recent window (last N messages or TTL) if memory is a concern — an unbounded Set grows without limit in long-lived sessions.

• ID: community-realtime.message-delivery.client-deduplication

• Severity: high

• What to look for: Count all client message handlers. For each, classify whether deduplication logic tracks received message IDs. Enumerate the deduplication mechanisms: Set, Map, or database-backed tracking.

• Pass criteria: The client generates a unique ID for outgoing messages and tracks at least 1 set of received IDs to deduplicate. Duplicate receipts are ignored with 0 duplicate renders.

• Fail criteria: No deduplication logic, or IDs are not client-generated.

• Skip (N/A) when: Never — deduplication prevents duplicate renders from network retries.

• Detail on fail: "Client message handler has no deduplication. Network retries could cause the same message to be rendered twice."

• Remediation: Implement client-side deduplication:

  const receivedMessageIds = new Set<string>();
  
  socket.on('message', (msg: Message) => {
    if (receivedMessageIds.has(msg.id)) {
      return; // Ignore duplicate
    }
    receivedMessageIds.add(msg.id);
    renderMessage(msg);
  });
  

  When sending, include a client-generated ID:

  const sendMessage = (content: string) => {
    const clientId = nanoid();
    socket.emit('send_message', { clientId, content });
  };