python删除重复数据

def odd_num(list1):
    list2 = []
    count = 0
    for i in list1:
        if i not in list2:
            list2.append(i)
    list2.sort()
    print(list2)
    for j in list2:
        print(j)
        if list2[j-1] == list2[j] - 1:
            count += 1
            print("count = ", count)
        elif count >= 2:
            print(list2[j-count-1], "-",  list2[j-1])
            count = 0



list1 = [1, 2, 3, 4, 1, 3, 6, 7, 8, 9, 11, 11]
odd_num(list1)

输入[1, 2, 3, 4, 1, 3, 6, 7, 8, 9, 11, 11]
输出需要是[1 - 4, 6 - 9, 11]
我这里应该怎么改进

你为什么不用二维数组记录呢？[[1,4],[6,9],[11,11]] ，先得到这样的结果，然后如果里面两个数相同，只输出一个数