title: How to define array typescript interface with optional unique strings?
body: <p>I'd like to define an interface with optional string values. Something like:</p>
  
  <pre><code>interface IEntity {
      values: ['RemainingUnits', 'ActualUnits', 'PlannedUnits']
  }
  </code></pre>
  
  <p>But with this interface I have problems:</p>
  
  <pre><code>const entity0: IEntity = { values: ['PlannedUnits'] }; // =&gt; Error
  const entity1: IEntity = { values: ['RemainingUnits', 'ActualUnits'] }; // =&gt; Error
  const entity2: IEntity = { values: ['PlannedUnits', 'RemainingUnits', 'ActualUnits'] }; // =&gt; Error
  </code></pre>
  
  <p>So are there ways to write the correct interface to avoid errors above?</p>
  
  <p>And perfectly without duplicated strings and not empty</p>