[faucet] can have duplicate entries in whitelist

[brenzi]

not sure if this is a problem, but we can add a cid multiple times:

./target/release/encointer-client-notee create-faucet //Alice Testfaucet2 100000000000000 1000000000000 sqm1v79dF6b sqm1v79dF6b

yielding

encointerFaucet.faucets: Option<EncointerPrimitivesFaucet>
[
  [
    [
      5Fy41AupevjQeURN1vVYyqJCjqXLvqPHCUhoRerqEEfVQQLt
    ]
    {
      name: Testfaucet2
      purposeId: 3
      whitelist: [
        {
          geohash: sqm1v
          digest: 0xf08c911c
        }
        {
          geohash: sqm1v
          digest: 0xf08c911c
        }
      ]
      dripAmount: 1,000,000,000,000
      creator: 5GrwvaEF5zXb26Fz9rcQpDWS57CtERHpNehXCPcNoHGKutQY
    }
  ]
]