Use epoch for attestation subnet seen-ness.

specs/phase0/p2p-interface.md

@@ -316,7 +316,7 @@ Attestation subnets are used to propagate unaggregated attestations to subsectio
     - The attestation's committee index (`attestation.data.index`) is for the correct subnet.
     - `attestation.data.slot` is within the last `ATTESTATION_PROPAGATION_SLOT_RANGE` slots (within a `MAXIMUM_GOSSIP_CLOCK_DISPARITY` allowance) -- i.e. `attestation.data.slot + ATTESTATION_PROPAGATION_SLOT_RANGE >= current_slot >= attestation.data.slot` (a client MAY queue future attestations for processing at the appropriate slot).
     - The attestation is unaggregated -- that is, it has exactly one participating validator (`len([bit for bit in attestation.aggregation_bits if bit == 0b1]) == 1`).
-    - There has been no other attestation seen on an attestation subnet that has an indentical `attestation.data.slot` and participating validator index.
+    - There has been no other attestation seen on an attestation subnet that has an indentical `attestation.data.target.epoch` and participating validator index.
     - The block being voted for (`attestation.data.beacon_block_root`) passes validation.
     - The signature of `attestation` is valid.