title: Validating list elements are unique in marshmallow
body: <p>I would like to understand if I can use <strong>marshmallow</strong> validate function to check whether all elements in a list are unique.</p>
  <p>I have the following schema:</p>
  <pre><code>from marshmallow import Schema, fields, validate
  
  class PaymentSchema(Schema):
      ...
      currencies = fields.List(
          fields.String(
              required=True,
              validate=[
                  validate.OneOf([&quot;USD&quot;, &quot;EUR&quot;, &quot;AUS&quot;, &quot;GBP&quot;]),
                  validate.Length(min=1, max=4)
              ]
          )
      )
  </code></pre>
  <p>Is there any possibility to guarantee that given a list of currencies with duplicated currencies (<code>[&quot;USD&quot;, &quot;EUR&quot;, &quot;EUR&quot;]</code>) raises a <em>ValidationError</em>?</p>
  <p>Thanks!</p>