df = data.frame(a = c(1,2,3,4,5), b = c(1,2,2,4,5))
duplicated(df, fromLast = TRUE)

mutate(df, b = replace(b, duplicated(.data$b, fromLast = TRUE), NA))

mutate(df, b = replace(b, duplicated(.data$b, fromLast = TRUE), NA))