difference between unique() and !duplicated()

a <- read.table(textConnection("x y   z

do.call('rbind', by(a, a$x, function(.sub){

Yesterday I spend the whole day struggling on how to get
the maximum value of "y" for every unique value of "x"
from the dataframe "test". In the R Book (Crawley, 2007)
an example of this can be found on page 121. I tried to do
it this way, but I failed.

In the end, I figured out how to get it working (first
order, and afterwards use !duplicated()). My question is:
why does it not work with the unique() function on p. 121
(
i.e. test[rev(order(x)),][unique(y),]) ?

As a simple example, I used to following syntax:

x <- c("A","A","B","B","C","C","D")
y <- c(1,2,1,1,2,3,1)
z <- c("yes","yes","no","yes","no","no","no")
test <- data.frame(x,y,z)
test

  x y   z
1 A 1 yes
2 A 2 yes
3 B 1  no
4 B 1 yes
5 C 2  no
6 C 3  no
7 D 1  no

test[rev(order(test$y, test$z)),][unique(test$x),]

  x y   z
6 C 3  no
2 A 2 yes
5 C 2  no
4 B 1 yes

# this clearly does not give a unique value for x, since
there are 2 C's and no D!

test[rev(order(test$y, test$z)),][!duplicated(test$x),]

  x y   z
6 C 3  no
5 C 2  no
1 A 1 yes
3 B 1  no

# this also doesn't work
# then I thought, maybe first use the order() function,
then unique()

test[rev(order(test$y, test$z)),]

  x y   z
6 C 3  no
2 A 2 yes
5 C 2  no
4 B 1 yes
1 A 1 yes
7 D 1  no
3 B 1  no

test1 <- test[rev(order(test$y, test$z)),]
test1[unique(test1$x),]

  x y   z
5 C 2  no
6 C 3  no
2 A 2 yes
4 B 1 yes

# still no unique values for x

test1[!duplicated(test1$x),]

  x y   z
6 C 3  no
2 A 2 yes
4 B 1 yes
7 D 1  no

# finally I get unique values for x, for the maximum value
of y (and z). But why does this not work when giving the
order() and !duplicated() command simultaneously?
And why does only !duplicated() work, and not unique()?