What does this achieve? It just says "add handler", does this mean it is the fix for#668 and#777?

Yes. I've just PR the code from issue#668 which should fix error with code 32.

This is very weird code.

@CheshireCaat can you review?

I think there is a more correct way to get rid of this error. How can I repeat it to make sure that this solution is right?

Okay, I looked at issues related to this problem, I think we need a more global solution for working with parallel requests as well.
Maybe it will be better to add something like SemaphoreSlim to theSendRequestAsync orRequestWithDcMigration?

Now the code from the example below will throw errors

awaitTask.WhenAll(Enumerable.Range(1,5).Select(arg=>Client.CheckUsernameAsync("CheshireCaat")))

I tested it also with modifiedRequestWithDcMigration withSemaphoreSlim - it works fine.
Can you check your error by adding Semaphore? If this helps, I think it's better to do it that way.

@n3nikita ping