Duplicate messages rendered in UI when writing custom data parts to the stream before calling agent.stream

[avi1989]

I have a workflow that reads data from various sources, streams them to the UI and then invokes the DurableAgent in a multi-chat workflow. I'm noticing that any messages that use the writable and are streamed before we invoke agent.stream are duplicated in the UI. The stream endpoint is only sending the data one time, but useChat is rendering them twice.
This issue has been happening since we upgraded WDK to 4.0.1-beta.49.

This can be reproduced by running flight-booking-app in my fork of vercel/workflow-examples.

I tried implementing the same changes with @workflow/ai 4.0.1-beta.48 and I did not see the duplicate message here

Screenshot without duplicate message

Screenshot with duplicate message

[avi1989]

I think this has something to do with the "start" chunk. Setting sendStart to false prevents the duplicates from showing up, but I don't know the issues that can be caused by doing that.