[BUG] Cowork (desktop): one task submission creates two sessions ~15 s apart, both execute the same prompt

What happened

One submission of a new Cowork task (a single paste of a ~30 KB markdown brief into the new-task box of the Claude desktop app, cloud/remote Cowork session linked to this Mac) created two sessions that both started executing the same brief. This is the fourth time I have seen it (2 Aug, 14 Sep, 25 Sep, 4 Oct 2026). On two of the earlier occasions both sessions wrote to a production database before anyone noticed.

Evidence (desktop app main.log, local time UTC+10)

2026-10-04 22:43:04 [info] LocalAgentModeSessions.grantRemoteSessionFolders: cse_017tGkfA4PMVUxd8nsvptB8y [1]
2026-10-04 22:43:19 [info] LocalAgentModeSessions.grantRemoteSessionFolders: cse_011YwGB8je5zdU77LBbKeYe9 [1]

Each of the two is preceded by an identical, complete creation sequence (getVMStorageSubpath, Config file written, using oauth config, cached token lookup). There is no error, retry or timeout line between them at info level. Both sessions received the same first user message and the same connected folder.

The same pairing occurred earlier the same day, 16 s apart:

2026-10-04 17:20:35 ... grantRemoteSessionFolders: cse_01AzaQyuDnhCZ6VzJW6ooaua [1]
2026-10-04 17:20:51 ... grantRemoteSessionFolders: cse_0138Y6wwULNZntGJDr5b3ZgD [1]

Expected

One submission creates one session. If session creation is retried client-side, the retry should carry an idempotency key so the server returns the first session rather than creating a second.

Notes

• The fixed ~15 s gap with no user action looks like a client re-submit when the first create is not acknowledged in time. I cannot see the trigger in the logs, so that is an inference. The app was busy (about 14 Cowork sessions created that day) and the paste was large.
• Nothing in the UI indicated a second session had been created; it only showed up in the sidebar.
• Possibly related: #31685 (claude.ai/code: every new session created twice).

Environment

• Claude desktop 2.19675.0, macOS 26.6.2, Apple Silicon MacBook Air
• Cowork, cloud session linked to the computer, session attached to a Project, one connected folder
• Session ids above can be used to look up the server side