unique drops duplicates so that each element in the result is unique (only appears once):
x = c(2, 1, 1, 2, 1) unique(x) # 2 1
Values are returned in the order they first appeared.
duplicated tags each duplicated element:
duplicated(x) # FALSE FALSE TRUE TRUE TRUE
anyDuplicated(x) > 0L is a quick way of checking whether a vector contains any duplicates.