Repeated requests to `POST {helper}/tasks/{task-id}/aggregate_shares` are aborted

[cjpatton]

If we detect a batch overlap, we abort: https://github.com/cloudflare/daphne/blob/main/daphne/src/roles/helper.rs#L199-L209

But if the batch interval hasn't changed, we're meant to handle this idempotently: https://datatracker.ietf.org/doc/html/draft-ietf-ppm-dap-09#section-4.6.2-16