Remove repeated elements from a column in r

Use duplicated

dates$A <- replace(dates$A, duplicated(dates$A), NA)

Or update the original object

dates$A[duplicated(dates$A)] <- NA

if these are based on adjacent values, use rle

dates$A <- replace(dates$A, duplicated(with(rle(dates$A), 
     rep(seq_along(values), lengths))), NA)

If we want to remove the rows

subset(dates, !duplicated(A))

Here is how you can implement akrun's solution with mutate:

library(dplyr)
dates %>% 
  mutate(A = replace(A, duplicated(A), NA))

  A         B
1  2021 something
2  2022 something
3    NA something
4    NA something
5    NA something
6    NA something
7    NA something
8    NA something
9    NA something
10 2023 something
11   NA something
12   NA something
13   NA something
14 2024 something
15 2025 something
16   NA something
17   NA something
18   NA something