Subscribing to attestation subnets for aggregating does not work correctly

[povi]

Description

If [/eth/v1/validator/beacon_committee_subscriptions](https://ethereum.github.io/beacon-APIs/#/Validator/prepareBeaconCommitteeSubnet) request contains multiple aggregator subscriptions to the same attestation subnet at different slots, it respects only the last subscription per subnet:

E.g. if there request contains these subscriptions:

[
  {
    "validator_index": "1",
    "committee_index": "1",
    "committees_at_slot": "2",
    "slot": "2",
    "is_aggregator": true
  },
  {
    "validator_index": "2",
    "committee_index": "2",
    "committees_at_slot": "2",
    "slot": "7",
    "is_aggregator": true
  },
  {
    "validator_index": "3",
    "committee_index": "1",
    "committees_at_slot": "2",
    "slot": "12",
    "is_aggregator": true
  }
]

it will only subscribe to attestation subnets for aggregating at slots 7 and 12, ignoring slot 2.