Python基础-List找重复数

请从L=[1,10,20,50,20,20,1]中找出重复数。

L=[1,10,20,50,20,20,1]
L1=[]
for i in L:
    if(L.count(i)>1):
        L1.append(i)
L2=[]
for i in L1:
    if i not in L2:
        L2.append(i)
print L2