Guard that rejects duplicate Allocate on the same client 5-tuple.

What problem this solves

Only one allocation may exist per client 5-tuple. If the client retries the same Allocate transaction, this step replays the success response; if a different transaction arrives while an allocation is active, it returns 437.

Second step in the @allocation pipeline, before Authenticates.

Internal

Pipeline action only; not part of the public application API.

RFCs

• RFC 5766 - one allocation per 5-tuple (pt.6.2)