Fill NA only between 2 same non-NA values

Question

I would like to fill NAs of a pandas series by the non-NAs values, but only if the non-NA values padding the NA values are the same. Is there any clever fast solution? I know I could write a function which I would use in the itterrows setting but I am operating with millions of rows and need a faster solution.

Example of Input:

0    NaN
1    1
2    NaN
3    1
4    NaN
5    2
6    NaN
7    NaN
8    2
9    NaN

Output:

0    NaN
1    1
2    1
3    1
4    NaN
5    2
6    2
7    2
8    2
9    NaN