type Check<A, E = never, R extends unknown[] = []> = A extends [infer F, ...infer T]
  ? Check<T, E | F, [...R, [F] extends [E] ? never : F]>
  : R;

function uniqueItems<const T>(items: Check<T> & T): T {
  return items;
}

// hack for TS 5.2.x and older:
// function uniqueItems<T extends readonly [] | [V, ...V[]], V extends PropertyKey | boolean | bigint | null | undefined>(items: Check<T> & T): T {
//   return items;
// }

Solution by alexandroppolus #35024

type UniqueItems<T, M = never> = T extends [infer F, ...infer R] ? F extends M ? never : [F, ...UniqueItems<R, M | F>] : []

function uniqueItems<const T>(items: T extends UniqueItems<T> ? T : UniqueItems<T>) {
  return items
}

Solution by Mantou1233 #34510

type Ensure<T,  _P extends unknown[] = []> =
  T extends [ infer F, ...infer R ]
    ? Ensure<R, [ ..._P, F extends _P[number] ? never : F ]>
    : _P extends T ? _P : _P

function uniqueItems<const T>(items: Ensure<T>) {
  return items
}

Solution by lvjiaxuan #33600

type UniqueItemsTuple<T>
  = {[I in keyof T]: I extends `${number}`
    ? I extends ExtractSmallest<keyof {[J in keyof T & `${number}` as T[J] extends T[I] ? J : never]: J}> ? T[I]
    : { error: `Item duplicated at position ${I}` }
  : T[I]}

type ExtractSmallest<N extends string> = keyof {[I in N as [keyof {[J in Exclude<N, I> as `${GreaterThan<I, J>}` & `true`]: never}] extends [never] ? I : never]: never}

type GreaterThan<X extends string, Y extends string, Gt extends boolean | undefined = undefined>
  = X extends Y ? Gt extends undefined ? false : Gt
  : `${X} ${Y}` extends `${infer A}${infer C} ${infer B}${infer D}` ? GreaterThan<C, D, Gt extends true | false ? Gt : A extends B ? Gt : '9876543210' extends `${any}${X}${any}${Y}${any}` ? true : false>
  : X extends '' ? false : true

function uniqueItems<const T>(items: T extends UniqueItemsTuple<T> ? T : UniqueItemsTuple<T>) {
  return items
}

Playground

Solution by teamchong #33165

type GenerateHintTuple<T, _U = never> = T extends [infer First, ...infer Rest]
  ? [First extends _U ? never : First, ...GenerateHintTuple<Rest, First | _U>]
  : []

function uniqueItems<const T>(items: T extends GenerateHintTuple<T> ? T : GenerateHintTuple<T>) {
  return items
}

Solution by Sun79 #33118