React.StrictMode setState function is getting called twice

[delwyn]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
Calling setState with a function gets called twice on a components that is wrapped in React.StrictMode.

CodeSandbox (https://codesandbox.io/s/zl9rv0mmkx)

class Example extends React.Component {
  componentDidMount() {
    this.setState(() => {
      console.log(`Set state calls in ${this.props.name}`);
    });
  }

  render() {
    return <div>{this.props.name}</div>;
  }
}

const App = () => (
  <div>
    <React.StrictMode>
      <Example name="StrictMode" />
    </React.StrictMode>
    <Example name="Normal" />
  </div>
);

Will output

Set state calls in StrictMode 
Set state calls in StrictMode 
Set state calls in Normal 

What is the expected behavior?
setState function should only be called once in StrictMode

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?
React: 16.4.0, 16.3.2
Browsers: Chrome, Firefox, Safari

[iamdustan]

This is intentional and why StrictMode exists. The double-invoking only happens in development mode.

https://reactjs.org/docs/strict-mode.html#detecting-unexpected-side-effects