Pandas foward fill in between – same values

I am trying to do the following:

Supposing I have the following column on pandas, where there will be always two values that are equal in sequence.

l = [np.nan, np.nan, 10, np.nan, np.nan, np.nan, 10, np.nan, 4, np.nan, 4, 5, np.nan, 5, np.nan,  2, np.nan, 2, 1, 1]

How can I fill NaN values only in between the interval of similar values ?

expected output:

[np.nan, np.nan, 10, 10, 10, 10, 10, np.nan, 4, 4, 4, 5, 5, 5, np.nan,  2, 2, 2, 1, 1]

I could only find this answer, which is not the same problem:

Stackoverflow Question

>Solution :

Solution with ffill and bfill

f = df['col'].ffill()
b = df['col'].bfill()

df['col'].mask(f == b, f)

0      NaN
1      NaN
2     10.0
3     10.0
4     10.0
5     10.0
6     10.0
7      NaN
8      4.0
9      4.0
10     4.0
11     5.0
12     5.0
13     5.0
14     NaN
15     2.0
16     2.0
17     2.0
18     1.0
19     1.0
Name: col, dtype: float64