Вот один простой способ сделать это -
# for numeric vector
x <- c(1:8, 4:10)
# [1] 1 2 3 4 5 6 7 8 4 5 6 7 8 9 10
x[ave(x, x, FUN = seq_along) == 1]
# [1] 1 2 3 4 5 6 7 8 9 10
# for character vector
x <- as.character(iris$Species)
x[ave(x, x, FUN = seq_along) == 1]
# [1] "setosa" "versicolor" "virginica"