CheckRepeatedTuple

Problem Link

Problem

Check whether a tuple contains duplicate elements. Return true if there are any duplicates, false otherwise.

type T0 = CheckRepeatedTuple<[1, 2, 3]>      // false
type T1 = CheckRepeatedTuple<[1, 2, 1]>      // true
type T2 = CheckRepeatedTuple<[1, 2, 3, '3']> // false

Solution

type Includes<T extends unknown[], U> =
  T extends [infer First, ...infer Rest]
    ? [First] extends [U]
      ? [U] extends [First]
        ? true
        : Includes<Rest, U>
      : Includes<Rest, U>
    : false

type CheckRepeatedTuple<T extends unknown[]> =
  T extends [infer First, ...infer Rest]
    ? Includes<Rest, First> extends true
      ? true
      : CheckRepeatedTuple<Rest>
    : false

How it works:

1. For each element First, check whether it already appears in the remaining elements Rest using Includes.
2. If found, return true immediately.
3. Otherwise, recurse on Rest.
4. If we exhaust the tuple without a duplicate, return false.

Includes uses the bidirectional [X] extends [Y] trick for exact equality to avoid issues with boolean (which is true | false).

Key Takeaways

• This is essentially an O(n²) set-membership check implemented recursively.
• Checking Rest (tail) rather than Seen (head) avoids needing a separate accumulator.
• The exact-equality check [A] extends [B] && [B] extends [A] is important to distinguish 1 from number.