how to make sure two random numbers are not the same in javascript

I am learning React and Javascript, I am writing a small app that have 40 squares with different colors and every second 2 colors will randomly change, everything works fine but I have one minor problem. How can I change my code so that if two numbers are the same one of them will be regenerated? I tried to use if like this

import React from 'react';
import ReactDom from 'react-dom';

const totalColor=40;
const Box=(props)=>{...}

class RandomColorApp extends React.Component{
    constructor(props){
        super(props);   
        const boxes = new Array(totalColor).fill().map(this.randomColor,this);
        this.state={boxes}

        setInterval(()=>{

           const randomIndex1=Math.floor(Math.random()*3)
           const randomIndex2=Math.floor(Math.random()*3)
           if(randomIndex1!==randomIndex2){
              console.log(randomIndex1,randomIndex2)
           }
        },1000);
    }

    randomColor(){...}
    render(){...}

}

RandomColorApp.defaultProps={colors:[...]}

ReactDom.render(<RandomColorApp/>,document.getElementById('app'));

the whole process will be delayed by 1 second before it regenerates new numbers, and also is there a way I can refactor the code so I do not have to repeat Math.floor(Math.random()*3) too much in case I need more than 2 random numbers, thank you verymuch