Unique

The solution

type Unique<
  T extends readonly unknown[],
  Acc extends unknown[] = [],
> = T extends readonly [infer H, ...infer R]
  ? Includes<Acc, H> extends true
    ? Unique<R, Acc>
    : Unique<R, [...Acc, H]>
  : Acc

The common wrong answer

type Unique<T extends readonly unknown[]> =
  T extends readonly [infer H, ...infer R]
    ? Includes<R, H> extends true
      ? Unique<R>
      : [H, ...Unique<R>]
    : []

This looks ahead instead of remembering. When an element recurs later it drops the *first* occurrence and keeps the last, so `[1, 2, 2, 3, 1]` becomes `[2, 3, 1]`. The elements are right, the order is not.

Line by line

1. Acc extends unknown[] = []

   A defaulted type parameter is how a type-level function carries state. The caller never passes it; each recursive call threads the growing result through.

2. Includes<Acc, H>

   The membership test runs against what has already been kept, not against what is still to come. That is the difference between keeping the first occurrence and keeping the last.

3. : Acc

   When the input is exhausted the accumulator *is* the answer, so it is returned directly rather than built up on the way out. This is a tail-recursive shape, and TypeScript handles it in less stack than the head-and-tail form.

Takeaway

An accumulator parameter turns a backwards-building recursion into a forwards-building one. It costs a type parameter and buys both correct ordering and a deeper recursion limit.