SOURCE

function duplicates(arr) {
    let duplicateObj = new Set()
    let result = []
    for(let i=0;i<arr.length;i++){
        console.log("current element",arr[i],"has is",duplicateObj.has(arr[i]))
        if(duplicateObj.has(arr[i])){
            result.push(arr[i])
        }else{
            duplicateObj.add(arr[i])
        }
    }

    let resultSet = new Set(result)
    return [...resultSet]
}

console.log(duplicates([1, 2, 4, 4, 3, 3, 1, 5, 3]))