title: Typescript - Duplicate identifier 'beforeEach'
body: <p>I just tried bootstraping a new project with create-react-app (1.5.2)</p>
  
  <pre><code>create-react-app test-ts --scripts-version=react-scripts-ts
  </code></pre>
  
  <p>and without touching anything, I 'yarn start' and I get the following :</p>
  
  <pre><code>&lt;project root&gt;/node_modules/@types/jest/index.d.ts
  (23,13): Duplicate identifier 'beforeEach'.
  </code></pre>
  
  <p>When I put </p>
  
  <pre><code>'skipLibCheck': true
  </code></pre>
  
  <p>in tsconfig.json, everything transpiles without any issue. </p>
  
  <p>I obviously tried on another ubuntu machine and it works just fine without the 'skipLibCheck': true in tsconfig.json</p>
  
  <p>I already tried with a fresh version of node and npm, already tried</p>
  
  <pre><code>rm -rf ./node_modules
  (npm|yarn) install
  </code></pre>
  
  <p>The error is still there.
  What do you think could be wrong ?</p>
  
  <p>Here's my tsconfig.json:</p>
  
  <pre><code>{
     "compilerOptions": {
         "baseUrl": ".",
         "outDir": "build/dist",
         "module": "esnext",
         "target": "es5",
         "lib": ["es6", "dom"],
         "sourceMap": true,
         "allowJs": true,
         "jsx": "react",
         "moduleResolution": "node",
         "rootDir": "src",
         "forceConsistentCasingInFileNames": true,
         "noImplicitReturns": true,
         "noImplicitThis": true,
         "noImplicitAny": true,
         "strictNullChecks": true,
         "suppressImplicitAnyIndexErrors": true,
         "noUnusedLocals": true
    },
    "exclude": [
        "node_modules",
        "build",
        "scripts",
        "acceptance-tests",
        "webpack",
        "jest",
        "src/setupTests.ts"
    ]
  }
  </code></pre>