yupSmaple2.js

function validateDuplicatedEmail(email) {
  const { users } = this.from[1].value;
  if (users.length < 2) return true;

  let dupCount = 0;
  for (let i = 0; i < users.length; i += 1) {
    if (users[i].email === email) {
      dupCount += 1;
      if (dupCount > 1) {
        return false;
      }
    }
  }

  return true;
}

const schema = yup.object().shape({
  users: yup.array(
    yup.object().shape({
      name: yup.string().required('name is required.'),
      email: yup
        .string()
        .required('email is required.')
        .email('invalid email type.')
        .test('email-dup', 'duplicated email', validateDuplicatedEmail)
    }),
  ),
});