data = [10, 9, 10, 10, 9, 8] 
a = data.copy() # создаю поверхностную копию спиcка data
# в цикле перебираю значения data, а удаляю данные из списка а, его и вывожу
for i in range(len(data)): 
    if data.count(data[i]) == 1:
        a.remove(data[i]) 
print(a)

 [code]data = [1, 2, 3, 4, 5] 
# в цикле перебираю значения data, удаляю данные из списка, вывожу
for i in range(len(data)): 
    if data.count(data[i]) == 1:
        data.remove(data[i]) 
print(data)
Traceback (most recent call last):
  File "<pyshell#22>", line 2, in <module>
    if data.count(data[i]) == 1:
IndexError: list index out of range[/code]

 print([data.remove(data[i]) for i in range(len(data)) if data.count(data[i]) == 1])

 a = [data.remove(data[i]) for i in range(len(data)) if data.count(data[i]) == 1]
print(a)

 [i for i in data if data.count(i)-1]

 i for i in data if data.count(i)-1

 for i in data:
if data.count(i).....

 >>> l =[10,10,9,9, 8,9]
>>> d ={}
>>> for i in l:
	d.setdefault(i,0)
	d[i] += 1
>>> d
{10: 2, 9: 3, 8: 1}
>>> [ i for i in l if d[i] > 1]
[10, 10, 9, 9, 9]
>>> 

  
>>> lst = [1, 2, 3, 1, 2, 4, 'a', 'b', 'a']
>>> 
>>> seen = {}
>>> rep = set()
>>> n = 0
>>> 
>>> for e in lst:
...     if e not in rep:
...         if e in seen:
...             rep.add(e)
...             del seen[e]
...         else:
...             seen[e] = n
...             n += 1
... 
>>> seen
{3: 2, 4: 3, 'b': 5}
>>> rep
{1, 2, 'a'}
>>>

 from time import time
def f1(lst):
    return [i for i in lst if lst.count(i)-1]
def f2(lst):
    d = {}
    for i in lst:
        d.setdefault(i,-1)
        d[i] += 1
    return [i for i in lst if d[i]]
l = list(range(10000)) + [1,2,3]
t0 = time()
l1 = f1(l)
print(time() - t0,l1)
t0 = time()
l2 = f2(l)
print(time() - t0,l2)

 1.8144128322601318 [1, 2, 3, 1, 2, 3]
0.00427699089050293 [1, 2, 3, 1, 2, 3]

 data = [1, 2, 3, 1, 2, 4, 'a', 'b', 'a']
dict_data = {}
for i,x in enumerate(data):
    dict_data[x] = dict_data.get(x,0)+1
print(list(filter(lambda x : dict_data.get(x) > 1,data )))

 [1, 2, 1, 2, 'a', 'a']