Linear time data duplicate check temporary storage comparison between JavaScript arrays and objects. It appears using arrays as temporary storage would give you 2x faster performance when compared to using Object literals.

linearDataDuplicateChecker.js

let numbers = [];

for(let i = 0; i < 100000; i++){
    numbers.push(Math.round(Math.random()*100000))
}

function hashDuplicate(value){
    console.time("hash duplicate")
    let hashValues = {}
    for(let i = 0; i < numbers.length; i++){
        if(hashValues[i] == undefined){
            hashValues[i] = 1;
        } else{
            return true;
        }
    }
    console.timeEnd("hash duplicate")
    return false;
}

function arrayDuplicate(value){
    console.time("array duplicate")
    let arrayValues = [];
    for(let i = 0; i < numbers.length; i++){
        if(arrayValues[i] == undefined){
            arrayValues[i] = 1;
        } else{
            return true;
        }
    }
    console.timeEnd("array duplicate")
    return false;
}

console.log("hash about to start")
console.log("found a duplicate for", numbers[0], hashDuplicate(numbers[0]))
console.log("hash has ended")
console.log("array about to start");
console.log("found a duplicate for", numbers[0], arrayDuplicate(numbers[0]))
console.log("array has ended")