title: Typescript guide gives "Duplicate function implementation" warning
body: <p>I'm getting started with TypeScript and at moment I'm following the <a href="https://www.typescriptlang.org/docs/handbook/typescript-in-5-minutes.html" rel="noreferrer">TypeScript in 5 minutes</a> guide. I'm receiving a strange warning in Visual Studio Code when I hover the mouse over the <code>greeter</code> function name, as shown in the below image. The alert is:</p>
  
  <blockquote>
    <p>[ts] Duplicate function implementation.</p>
    
    <p>function greeter(person: Person): string (+1 overload)</p>
  </blockquote>
  
  <p><a href="https://i.stack.imgur.com/i2brP.png" rel="noreferrer"><img src="https://i.stack.imgur.com/i2brP.png" alt="Duplicate function implementation warning."></a></p>
  
  <p>But there is no other implementation of this unique function in my single file! When I run <code>tsc greeter.ts</code> all works fine and the js file is generated.</p>
  
  <p>The complete <code>greeter.ts</code> file:</p>
  
  <pre><code>interface Person {
      firstName: string;
      lastName: string;
  }
  
  function greeter(person: Person) {
      return "Hello, " + person.firstName + " " + person.lastName;
  }
  
  var user = { firstName: "Jane", lastName: "User" };
  
  console.log(greeter(user));
  </code></pre>
  
  <p>Why am I receiving this alert? How to solve it? I took a look in <a href="https://stackoverflow.com/questions/39689763/typescript-duplicate-function-implementation">this question</a>, but I believe it isn't related.</p>