What is the difference between a data structure that stores unique elements and a sequence of elements that allows repetitions?

A data structure that stores unique elements is called a set (Set). In a set, each element appears only once, and repetitions are not allowed.

A sequence of elements that allows repetitions is, for example, an array (Array) or a list (List). In these, elements can repeat and have an ordered sequence.

Example in Swift:

var uniqueSet: Set<Int> = [1, 2, 3, 3]
print(uniqueSet) // Outputs {1, 2, 3} — duplicate element removed

var array: [Int] = [1, 2, 3, 3]
print(array) // Outputs [1, 2, 3, 3] — repetitions are preserved

Thus, the main difference is the uniqueness of elements and the order of storage.