first seen

Definition

refers to the first instance of a Message received by any Witness or Watcher. The first-seen event is always seen, and can never be unseen. It forms the basis for Duplicity detection in KERI-based systems.
Source: Dr. S.Smith

Explanation

A "First seen" event in KERI refers to the first event received by a validator, such as a witness. The event is valid and fits the available tail sequence number in the validator's KEL and, therefore, is accepted into the validator's KEL. This rule has no effect on the timing of what has arrived in escrow. For example, in escrow, there can be garbage. Assuming a watched set of validators agree on the first-seen events and thus also agree on the KELs, the watchers of those validators will propagate only those first-seen events within microseconds.

The rule

From the perspective of a validator, the rule is "First seen, always seen, never unseen."

Key Compromise, Duplicity, and Recovery

Different validators might have a different first-seen number for the same originating transaction event. In the case of duplicitous (inconsistent) interaction events originating from the controller (of the current signing key(s)), which might not be discovered until after a key rotation, a recovery process involving judges and jury may be triggered. More here. Validators will not provide an outdated KEL or Event once an erroneous KEL has been corrected.