Duplicate Item Errors

### Describe the bug

After initial prompt during processing, I receive errors of this format:
Execution failed: CAPIError: 400 {"error":{"code":"bad_request","type":"websocket_error","message":"Duplicate item found with id fc_call_3L6mbkYELCQh2XxmHQlVommW. Remove duplicate items from your input and try again."}} (Request ID: 00000-63415e69-425e-45d5-8f8b-23bcb99c727d)

With a subsequent prompt, I see this output:
● Request failed (transient_bad_request). Retrying...

● Request failed (session_token_refresh). Retrying...

● Request failed (session_token_refresh). Retrying...
✗ Execution failed: CAPIError: 400 Duplicate item found with id fc_call_3L6mbkYELCQh2XxmHQlVommW. Remove duplicate items from your input and try again. (Request ID: 5CA2:F9304:7D0A:8E64:6A189465)

I have quit the session and resumed, cleared the session, and quit and started fresh. I have switched between GPT-5.4, Sonnet 4.6 and auto and still receive errors.

### Affected version

GitHub Copilot CLI 1.0.54

### Steps to reproduce the behavior

1. copilot cli in interactive mode
2. use an agent defined at user-level
3. run prompt

### Expected behavior

No duplicate request errors

### Additional context

macOS Tahoe 26.5
zsh shell
gh installed via homebrew

未指定原始檔案或測試。首先，在 macOS 上使用 zsh，透過使用者層級的 agent 於 Copilot CLI 互動模式中重現此失敗，然後比較初始 prompt 與後續 prompt；當這些 prompt 不再產生重複項目錯誤或重試迴圈時，即視為完成。

由索引模型根據 Issue 內容生成。