Issue understanding modification of variable in jupyter notebook

Question

I'm quite new to jupyter and just can't figure out how this work :

With these 3 blocks of code, a multiple execution of the 3rd one make n incremental, i.e. [1,2], then [1,3], [1,4], etc

n = [1,2]

--

def fonction(x): 
  y=x
  y[1]=x[1]+1
  return y

--

res = fonction(n)
print(res)

This is the minimal example i can provide. I don't understand why the variable n is modified though it's only the argument in the 3rd block (and the 2nd block isn't modifying the argument of the function !)

Thanks a lot for your patience explaining 101 jupyter to this old engineer, sincerely