let arr = [1,2,2,3,4,5,5,5,5,5,5,6,7,8,8,8,9];

function removeDups(arr) {
  let newArr = [];
  for(let i = 0; i<arr.length; i++) {
    if(arr[i] !== arr[i+1] && arr[i] !== arr[i-1]) newArr.push(arr[i]);
  }
  return newArr;
}

removeDups(arr);