message: [account]: when import, ensure address not exist
diff: @@ -104,6 +104,9 @@ where
  private_key: Vec<u8>,
  password: &str,
  ) -> Result<WalletAccount, Error> {
  + if self.contains(&address)? {
  + bail!("account with address {} already exists", &address);
  + }
  let private_key = Ed25519PrivateKey::try_from(private_key.as_slice())?;
  let key_pair = KeyPair::from(private_key);
  let account = WalletAccount::new(address, key_pair.public_key.clone(), false);
  