uniqueItems property

Whether or not all the items in this list must be unique.

For example, to define a schema for a list where all items must be unique:

final schema = ListSchema(
  items: Schema.string(),
  uniqueItems: true,
);

In this schema, a list like ['apple', 'banana', 'cherry'] would be valid, but ['apple', 'banana', 'apple'] would be invalid because it contains duplicate items. Note that the type of the items is also defined using the items property; uniqueItems only enforces the uniqueness of the items, not their type or value, which are handled by the corresponding schema in the items property.