MAF send duplicated mesage

[thangchung]

• https://github.com/thangchung/agent-engineering-experiment/blob/001-coffeeshop-ordering/speckit-coffeeshop/coffeeshop/frontend/src/app/api/copilotkit/route.ts#L28
• https://github.com/thangchung/agent-engineering-experiment/blob/main/DotNetClaw/DotNetClaw/ClawRuntime.cs#L84

We might need this dedup code after upgrade code to https://github.com/microsoft/agent-framework/releases/tag/dotnet-1.0.0-rc4

See: microsoft/agent-framework#3981