title: Babel 7 fails with single plugin saying "Duplicate plugin/preset detected."
body: <p>The plugin that fails is <a href="https://www.npmjs.com/package/@babel/plugin-transform-regenerator" rel="noreferrer"><code>@babel/plugin-transform-regenerator</code></a> (no marginal plugin, 1.6 milion downloads / week).</p>
  
  <p>This is my entire <code>.babelrc</code>:</p>
  
  <pre><code>{
    "presets": [],
    "plugins": [
      "@babel/plugin-transform-regenerator"
    ]
  }
  </code></pre>
  
  <p>When I try to transpile it with parcel using <code>parcel build source/main/index.html --no-source-maps --out-dir build</code> I get the following error: </p>
  
  <pre><code>/path/to/index.js: Duplicate plugin/preset detected.
  If you'd like to use two separate instances of a plugin,
  they need separate names, e.g.
  
  plugins: [
    ['some-plugin', {}],
    ['some-plugin', {}, 'some unique name'],
  ]
  
  at assertNoDuplicates (/.../node_modules/@babel/core/lib/config/config-descriptors.js:205:13)
  at createDescriptors (/.../node_modules/@babel/core/lib/config/config-descriptors.js:114:3)
  at createPluginDescriptors (/.../node_modules/@babel/core/lib/config/config-descriptors.js:105:10)
  at alias (/.../node_modules/@babel/core/lib/config/config-descriptors.js:63:49)
  at cachedFunction (/.../node_modules/@babel/core/lib/config/caching.js:33:19)
  at plugins.plugins (/.../node_modules/@babel/core/lib/config/config-descriptors.js:28:77)
  at mergeChainOpts (/.../node_modules/@babel/core/lib/config/config-chain.js:314:26)
  at /.../node_modules/@babel/core/lib/config/config-chain.js:278:7
  at buildRootChain (/.../node_modules/@babel/core/lib/config/config-chain.js:68:29)
  at loadPrivatePartialConfig (/.../node_modules/@babel/core/lib/config/partial.js:85:55)
  </code></pre>
  
  <p>Here are my versions from package.json:</p>
  
  <pre><code>"@babel/core": "^7.1.2",
  "@babel/plugin-transform-regenerator": "^7.0.0",
  </code></pre>
  
  <p>Any ideas?</p>