duplicated()

• duplicated() returns a logical vector indicating whether a each element in x is repeated in x

x <- sample(1:10, 10, replace = TRUE)
duplicated(x)

##  [1] FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE  TRUE

# vs.
match(x, unique(x))

##  [1] 1 2 3 1 4 5 6 7 8 1