Allow second sync committee message to propagate if first one stale

specs/altair/p2p-interface.md

@@ -140,6 +140,8 @@ def get_sync_subcommittee_pubkeys(state: BeaconState, subcommittee_index: uint64
 - _[REJECT]_ `contribution_and_proof.selection_proof` selects the validator as an aggregator for the slot -- i.e. `is_sync_committee_aggregator(contribution_and_proof.selection_proof)` returns `True`.
 - _[REJECT]_ The aggregator's validator index is in the declared subcommittee of the current sync committee --
   i.e. `state.validators[contribution_and_proof.aggregator_index].pubkey in get_sync_subcommittee_pubkeys(state, contribution.subcommittee_index)`.
+- _[IGNORE]_ The block being signed (`contribution_and_proof.contribution.beacon_block_root`) has been seen (via both gossip and non-gossip sources) (a client MAY queue sync committee contributions for processing once block is received)
+- _[REJECT]_ The block being signed (`contribution_and_proof.contribution.beacon_block_root`) passes validation.
 - _[IGNORE]_ A valid sync committee contribution with equal `slot`, `beacon_block_root` and `subcommittee_index` whose `aggregation_bits` is non-strict superset has _not_ already been seen.
 - _[IGNORE]_ The sync committee contribution is the first valid contribution received for the aggregator with index `contribution_and_proof.aggregator_index`
   for the slot `contribution.slot` and subcommittee index `contribution.subcommittee_index`
@@ -161,7 +163,9 @@ The following validations MUST pass before forwarding the `sync_committee_messag
 - _[IGNORE]_ The message's slot is for the current slot (with a `MAXIMUM_GOSSIP_CLOCK_DISPARITY` allowance), i.e. `sync_committee_message.slot == current_slot`.
 - _[REJECT]_ The `subnet_id` is valid for the given validator, i.e. `subnet_id in compute_subnets_for_sync_committee(state, sync_committee_message.validator_index)`.
   Note this validation implies the validator is part of the broader current sync committee along with the correct subcommittee.
-- _[IGNORE]_ There has been no other valid sync committee message for the declared `slot` for the validator referenced by `sync_committee_message.validator_index`
+- _[IGNORE]_ The block being signed (`sync_committee_message.beacon_block_root`) has been seen (via both gossip and non-gossip sources) (a client MAY queue sync committee messages for processing once block is retrieved).
+- _[REJECT]_ The block being signed (`sync_committee_message.beacon_block_root`) passes validation.
+- _[IGNORE]_ There has been no other valid sync committee message for the declared `slot` for the validator referenced by `sync_committee_message.validator_index`, unless the block being signed (`beacon_block_root`) matches the local head as selected by fork choice, and the earlier valid sync committee message does not match
   (this requires maintaining a cache of size `SYNC_COMMITTEE_SIZE // SYNC_COMMITTEE_SUBNET_COUNT` for each subnet that can be flushed after each slot).
   Note this validation is _per topic_ so that for a given `slot`, multiple messages could be forwarded with the same `validator_index` as long as the `subnet_id`s are distinct.
 - _[REJECT]_ The `signature` is valid for the message `beacon_block_root` for the validator referenced by `validator_index`.