title: Typescript: Duplicate vars across multiple files allowed?
body: <p>Not sure if this is a bug in Typescript (2.1.5) or intended behavior, but if I have two files declaring the same block-level var in the same folder:</p>
  <p><strong>a.js</strong></p>
  <pre><code>let x = 0;
  </code></pre>
  <p><strong>b.js</strong></p>
  <pre><code>let x = 1;
  </code></pre>
  <p>... and build it with <code>tsc</code>, I get:</p>
  <blockquote>
  <p>a.ts(1,5): error TS2451: Cannot redeclare block-scoped variable 'x'.</p>
  <p>b.ts(1,5): error TS2451: Cannot redeclare block-scoped variable 'x'.</p>
  </blockquote>
  <p>Is this supposed to happen? Or does Typescript have the concept of a 'default' namespace, shared across the entire project?</p>
  <p>I originally thought this may have something to do with <a href="https://www.typescriptlang.org/docs/handbook/declaration-merging.html" rel="noreferrer">Declaration Merging</a>, but the equivalent using explicit namespaces seems to work fine:</p>
  <p><strong>c.js</strong></p>
  <pre><code>namespace Test {
      let x = 0;
  }
  
  namespace Test {
      let x = 1;
  }
  </code></pre>