grouping_bunny.js

var k = [1,4,2,5,3,2,2,5,7,8,8,5,3,4,4,4,2,1]

function bundle(arr) {
  var t = {}
  arr.map(function (x) {
    if (!t[x]) t[x] = []
    t[x].push(x)
  })
  return Object.keys(t).reduce(function (memo, x) { return memo.concat([t[x]])}, [])
}

console.log(bundle(k))