incrementing elements of 2d list in python

Question

I've noticed something strange when trying to implement elements of a 2d list in python. Here is my code

n_dk=2*[5*[0]]
n_dk[0][0]+=1
print n_dk

I would expect the output to be

[[1,0,0,0,0],[0,0,0,0,0]]

but the actual output is

[[1,0,0,0,0],[1,0,0,0,0]]

can anyone tell me what I am doing wrong. btw I used a numpy array instead and it worked the way I wanted it to.