title: Is there an annotation for java validate if the value of a field in a List of Objects is duplicated?
body: <p>I have a List of Objects and each Object have an email, I'd like to validate if the email is duplicated in this list.</p>
  
  <pre><code>@PostMapping
  @ResponseStatus(HttpStatus.CREATED)
  Father create(@PathVariable String id,
          @Valid @RequestBody Father father) {
     ...
  }
  </code></pre>
  
  <p>Father will have a list of child:</p>
  
  <pre><code>private List&lt;Child&gt; childs;
  </code></pre>
  
  <p>Each child will have an email:</p>
  
  <pre><code>public class Child {
    ...
  
    @NotEmpty
    private String email;
  
    ...
  }
  </code></pre>
  
  <p>I'd like to validate if for example there is a request body with 2 child with the same email.</p>
  
  <p>Is it possible or only validating after receive and process the payload?</p>