I am just starting with ReactJS and tried solutions of other questions similar to this but no luck so far.

Here is my working code :

import React from 'react';

import ReactDOM from 'react-dom';





const Numbers = ['2', '4', '6', '8'];



const NumbersList = (props) => (

   <ul>

      {

          props.Numbers.map (

             number => <li key={number}>{number * 2}</li>

          )

      }

    </ul>

)

ReactDOM.render(<NumbersList Numbers = {Numbers} />, document.getElementById('root') )

But when I am passing Numbers Array as :

const Numbers = ['4', '4', '6', '8']

I am getting this error :

Warning: Encountered two children with the same key, 4. Keys should be unique so that components maintain their identity across updates.

So my Question is : What is the best way to give keys in this situation? And if I am using Number (as in above example) as Keys, what is the best solution to avoid this warning?

Thank You!