In gitgitgadget/git#54 (comment) you can see that GitGitGadget failed because it tried to serve two /submit commands at the same time.

The safer things would be to switch GitGitGadget from an Azure WebApp to an Azure Function that triggers an Azure Pipeline.

That way, we can also have a public record of the submission process, and we can even use the Checks API on GitHub to show when patch submission is in progress (and when it completed).