[BUG] Sent messages duplicate/repeat in Cowork desktop app conversation thread

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When a message is sent in the Cowork desktop app, the same message appears 2-3 times consecutively in the conversation thread. The message was only typed and sent once. The duplicates persist in the chat history — they do not disappear after a moment. This happens during normal use in an active long-running conversation. The conversation otherwise continues functioning normally (responses are generated correctly), but the duplicated user messages accumulate and make the thread confusing to read.

What Should Happen?

A sent message should appear exactly once in the conversation thread. Sending a message should never result in it appearing multiple times in the history.

Error Messages/Logs

Steps to Reproduce

1. Open the Cowork desktop app and start or resume a conversation
2. Type a message and send it
3. Observe the conversation thread — the sent message appears 2–3 times consecutively
4. Reproducible in long-running conversations; duplicates persist in history

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

N/A - Cowork desktop app (research preview)

Claude Code Version

N/A - Cowork desktop app (research preview)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_