const arr = [1,2,4,5,5,2,6,7]

const findUniqueElem = (arr) => {
    const obj = {}
    for(value of arr){
        if(obj[value]){
            delete obj[value]
        }else{
            obj[value] = true
        }
    }
    return Object.keys(obj)
}

console.log(findUniqueElem(arr))