let arr = [1, 2, 3, 5, 6, 4, 3, 2, 1, 1, 2, 3, 4, 5]

for(let i = 0; i < arr.length - 1; ++i) {
    for(let j = i + 1; j < arr.length; ++j) {
        if(arr[i] === arr[j]) {
            arr.splice(j--, 1)
        }
    }
}

console.log(arr)

let arr = [1, 2, 3, 5, 6, 4, 3, 2, 1, 1, 2, 3, 4, 5]

function dedupe(array) {
    let interim = []
    array.forEach(function(value) {
        // if(this.indexOf(value) === -1)
        if(!this.includes(value))
            interim.push(value)
    }, interim)
    return interim
}

console.log(dedupe(arr))

let arr = [1, 2, 3, 5, 6, 4, 3, 2, 1, 1, 2, 3, 4, 5, '5']

function dedupe(array) {
    let interim = {}
    array.forEach(function(value) {
        interim[value] = null
    })
    return Object.keys(interim).map(v => parseInt(v))
}

console.log(dedupe(arr))