deepblack

def chunkify(items, chunk_size):
    for i in range(0, len(items), chunk_size):
        yield items[i:i+chunk_size]


a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

for chunk in zip(*chunkify(a, 4)):
    print(*chunk)

var arr = [1, 2, 3, 4, 2, 5];

function findDuplicate(arr) {
  var obj = {};
  for(var i=0; i<arr.length; i++) {
    if (obj[arr[i]]) return arr[i];
    obj[arr[i]] = true;
  }
}

console.log('Повторяющееся число:', findDuplicate(arr));