Cowork (claude.ai web): a single uploaded image is delivered to the model twice, causing it to assert the user sent two

Not the CLI — this concerns **Cowork on claude.ai in the browser** (cloud sandbox session, model `claude-opus-5`). CLI-only required fields (CLI version, terminal, `/doctor` output) are not applicable.

## What happened

The user attached **one** PNG screenshot to a message. The turn arrived at the model with the image rendered **twice**, back to back, as two apparently separate attachments — while the accompanying `system-reminder` listed only **one** saved upload path:

```
The attached image is also saved at
"/root/.claude/uploads//2fa68b43-image.png"
```

`md5` of that single file confirmed one image; there was no second file on disk.

## Consequence

The duplication is indistinguishable, from the model's side, from the user deliberately attaching the same image twice. The model answered the user's question ("do you see what I did there?") by asserting they had sent the screenshot two times. The user had sent one. That is a confidently wrong statement about the user's own action, caused entirely by the transport, and it took two further turns to untangle.

This is worse than a cosmetic render bug: the model has no signal available to detect it. The upload list said one file; the message content said two images. Nothing in the turn marks which is authoritative.

## Expected

An attachment uploaded once appears once in the model's view of the turn. If a duplicate render is unavoidable, the image blocks should be reconcilable against the uploads manifest so the model can tell one attachment from two.

## Repro

1. Open a Cowork session on claude.ai in a browser.
2. Attach a single PNG (3200x1800 in this case, ~screenshot sized) with a short text message.
3. Send.

Observed intermittently — a later message in the same session with no attachment was unaffected. Not yet isolated to a file size or type.

## Related

Neighbouring but distinct: #80514 (desktop browser pane renders a duplicated ghost copy) is a UI-side duplication; this one is in what reaches the model.

Start by reproducing the single-PNG upload in a Cowork session on claude.ai and compare the model turn with the uploads manifest at /root/.claude/uploads//. Done means one uploaded image appears once to the model, or duplicate blocks can be reconciled with the one-file manifest.

索引モデルが issue の本文から書いたものです。