Execution failed: CAPIError: 400 {"error":{"code":"bad_request","type":"websocket_error","message":"Duplicate item found with id fc_call_ikBrgmi06fcJeO6uIpJJEQbJ. Remove duplicate items from your input and try again."}}

### Describe the bug

This started happening suddenly in the evening with the same Copilot CLI version and the same workflow.

Important detail:
- Plain chat still works.
- The failure only happens on the turn after a tool/function call.
- During the day, the same flow worked normally.
- In the evening, follow-up turns after tool calls started failing with:
"Duplicate item found with id fc_call_..."

This suggests a server-side regression or rollout affecting tool-call session continuation, rather than a local client version issue.

### Affected version

GitHub Copilot CLI 1.0.55. Run 'copilot update' to check for updates.

### Steps to reproduce the behavior

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

No source file or test is named. Start by reproducing the workflow with Copilot CLI 1.0.55, comparing plain chat with the turn after a tool call; investigate session continuation around the reported CAPIError. Done means follow-up turns after tool calls no longer fail with the duplicate-item error.

索引モデルが issue の本文から書いたものです。