Perspective key server matrix.org returns keys multiple times for some entries

[reivilibre]

This is an offshoot of #12736.

rei@lithium ~$ curl https://matrix.org/_matrix/key/v2/query -d '{"server_keys": {"matrix.arsenm.dev": {"ed25519:a_TzAM": {}, "ed25519:ads3pX": {}}}}' | jq .

{
  "server_keys": [
    {
      "old_verify_keys": {},
      "server_name": "matrix.arsenm.dev",
      "signatures": {
        "matrix.arsenm.dev": {
          "ed25519:a_TzAM": "oQgYwB7VF/PAc0Y/VeNBKIAeNGxGnKKdamacfqpmQLfm3IGjAUSDBwlamlZRgAT7Cp9yb1dqVPxQcfB2akWqBg"
        },
        "matrix.org": {
          "ed25519:auto": "WMwuXWnDJuNbzCA3Pdc0ZfBgg0azw3SYl6SsksUrQXyKtu6uGAuiAOIjjXcuQP6hFB14ftgN6I8vkAd4sWqXDQ"
        }
      },
      "tls_fingerprints": [],
      "valid_until_ts": 1627273239740,
      "verify_keys": {
        "ed25519:a_TzAM": {
          "key": "4FvovoVi9G7MIQuLtf415BM2ZkZoyR7XR7wpBHXSL9w"
        }
      }
    },
    {
      "old_verify_keys": {
        "ed25519:a_TzAM": {
          "expired_ts": 1601024554498,
          "key": "4FvovoVi9G7MIQuLtf415BM2ZkZoyR7XR7wpBHXSL9w"
        }
      },
      "server_name": "matrix.arsenm.dev",
      "signatures": {
        "matrix.arsenm.dev": {
          "ed25519:ads3pX": "TIwJM/GR0MOjIgLozTB0KexowbuMG8DRGu9DAQbMcVDAgUNwWVglce/9LPTrfq+g7qj73Ml3mAaUj9MBD6cmDQ"
        },
        "matrix.org": {
          "ed25519:auto": "xuIzHyUzHqz7NL5t34GTCXSSh/rxhVOxktzJxT+LLRxY+9FlHQOJoRUPZJnBY2QjzgHmhWW3+JAoKOcoIlZQBw"
        }
      },
      "valid_until_ts": 1627788036360,
      "verify_keys": {
        "ed25519:ads3pX": {
          "key": "TGOiZYXHMEWCD6rgqiJhdrg7O4nN488jDPcj/V9b0xI"
        }
      }
    }
  ]
}

ed25519:a_TzAM appears multiple times. There are multiple array entries for the same server name.

I'm not clear on how this is meant to work, but it seems suspicious and in any case it was causing #12736 (CardinalityViolation errors due to the keys being duplicated).

We should investigate to see whether this is unwanted and whether we should fix the perspective server to not return these duplicates.