Avoid generating redundant claims after initial confirmation

Avoid generating redundant claims after initial confirmation

These claims will never be valid as a previous claim has already
confirmed. If a previous claim is reorged out of the chain, a new claim
will be generated bypassing the new behavior.

While this doesn't change much for our existing transaction-based
claims, as broadcasting an already confirmed transaction acts as a NOP,
it prevents us from yielding redundant event-based claims, which will be
introduced as part of the anchors patchset.