## What happened

On the `/v1/messages` → Responses API bridge, `AnthropicResponsesStreamWrapper` emits **two** `message_start` events on every stream:

1. `__anext__` eagerly emits `message_start` on the first pull (fallback for upstreams that never send `response.created`), setting `_sent_message_start = True`. 2. When the upstream then sends `response.created`, `_process_event` appends **another** `message_start` without checking `_sent_message_start`.

Since the eager fallback always runs before the first upstream event is processed, every stream that includes `response.created` (i.e. the normal case) gets a duplicate.

[Anthropic's streaming contract](https://docs.claude.com/en/api/messages-streaming) specifies exactly one `message_start` per stream.

## Reproduction

Drive the wrapper with any normal event sequence:

```python import asyncio from litellm.llms.anthropic.experimental_pass_through.responses_adapters.streaming_iterator import ( AnthropicResponsesStreamWrapper, )

async def upstream(): yield {"type": "response.created"} yield {"type": "response.completed", "response": {"status": "completed"}}

async def main(): wrapper = AnthropicResponsesStreamWra…