Behaviour update with simultaneous polling requests

[Nicolas Aillery]

The first polling request must be favored over the newest ones. The newest ones must be responded with an error.

-
The Client sent simultaneous requests to the User Questioning Polling Endpoint for the same <spanx style="verb">question_id</spanx>. This error is responded to oldest requests. The last request is processed normally.
+
The Client sent simultaneous requests to the User Questioning Polling Endpoint for the same <spanx style="verb">question_id</spanx>. This error is responded to newer requests. The oldest request is processed normally.

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[Bjorn Hjelm]

Approved.