중복되는 수

arr = [1,2,3,3,3,4,4]

def solution(arr):
  answer = []

  for v in arr:
    if(not answer) or (v not in list(zip(*answer))[1]):
      answer.append([1,v])
    else:
      for i in range(len(answer)):
        if answer[i][1] == v:
          answer [i][0] += 1

  answer = [cnt for cnt, value in answer if cnt > 1]
  if answer : return answer
  else : return [-1]