print only duplicate items in a list

list_a = [1,2,1,3,4,5,5,6,6,4,2]
size = len(list_a)
new_list = []
for i in range(size):
k = i+1
for j in range(k, size):
if list_a[i] == list_a[j] and list_a[i] not in new_list:
new_list.append(list_a[i])

print(new_list)