R - How to fill in values in NA, but only when ending value is the same as the beginning value?

Question

I have the following example data:

Example <- data.frame(col1 =c(1, NA, NA, 4, NA, NA, 6, NA, NA, NA, 6, 8, NA, 2, NA))

col1
1
NA
NA
4
NA
NA
6
NA
NA
NA
6
8
NA
2
NA

I want to fill the NAs with value from above, but only if the NAs are between 2 identical values. In this example the first NA gap from 1 to 4 should not be filled with 1s. But the gap between the first 6 and the second 6 should be filled, with 6s. All other values should stay NA. Therefore, afterwards it should look like:

col1
1
NA
NA
4
NA
NA
6
6
6
6
6
8
NA
2
NA

But in reality I do not have only 15 observations, but over 50000. Therefore I need a efficient solution, which is more difficult than I thought. I tried to use the Fill function but was not able to come up with a solution.

Answer 1

One dplyr and zoo option could be:

df %>%
    mutate(cond = na.locf0(col1) == na.locf0(col1, fromLast = TRUE),
           col1 = ifelse(cond, na.locf0(col1), col1)) %>%
    select(-cond)

   col1
1     1
2    NA
3    NA
4     4
5    NA
6    NA
7     6
8     6
9     6
10    6
11    6
12    8
13   NA
14    2
15   NA