l1 = ['b','c','d','c','a','a']
l2 = list(set(l1))
print(l2)

用list类的sort方法:

l1 = ['b','c','d','c','a','a']
l2 = list(set(l1))
l2.sort(key=l1.index)
print(l2)

也可以这样写:

l1 = ['b','c','d','c','a','a']
l2 = sorted(set(l1),key=l1.index)
print(l2)

也可以用遍历：

l1 = ['b','c','d','c','a','a']
l2 = []
for i in l1:
    if not i in l2:
        l2.append(i)
print(l2)