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Find duplicates in array

Q: Find duplicates in array

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Easy > **How to read this doc:** a real, easy-to-overlook detail this question tests — the CORRECT real order of the duplicate VALUES themselves in the output — was verified directly: `findDuplicates([3,1,3,2,1])` correctly produced `[3,1]`, matching the real order in which `3` and `1` FIRST appeared in the input, not an arbitrary or sorted order. ## 1. The problem, restated Identify every DISTINCT value that appears more than once in the array, returning each one EXACTLY ONCE in the output (regardless of how many total times it actually repeated) — in the real order each duplicate value FIRST appeared in the original input. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Each duplicate appears once in the output? | Yes — the output identifies WHICH values are duplicated, not every extra repeated occurrence. | | Real output order?

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The problem

Problem, as an interviewer would state it: "Implement findDuplicates(arr) — returning every element that appears MORE THAN ONCE in the array, with each duplicate value appearing exactly ONCE in the output."

Examples:

Code

findDuplicates([1,2,3,2,4,3,3]); // [2,3]

Clarifying questions expected:

Should each duplicate VALUE appear exactly once in the output, regardless of how many total times it repeated in the input?
What real, correct order should the duplicate values themselves appear in — their own first-occurrence order, or any order?
Does this need to also report HOW MANY times each duplicate appeared, or just identify which values are duplicated?

Code / implementation expected: Yes — real, direct proof of correct duplicate detection, confirmation an already-unique array correctly produces an empty result, and confirmation of the output's own real order.

js-coding
utility
algorithm

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Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Easy

How to read this doc: a real, easy-to-overlook detail this question tests — the CORRECT real order of the duplicate VALUES themselves in the output — was verified directly: findDuplicates([3,1,3,2,1]) correctly produced [3,1], matching the real order in which 3 and 1 FIRST appeared in the input, not an arbitrary or sorted order.

1. The problem, restated

Identify every DISTINCT value that appears more than once in the array, returning each one EXACTLY ONCE in the output (regardless of how many total times it actually repeated) — in the real order each duplicate value FIRST appeared in the original input.

2. Clarifying questions a strong candidate asks

Question	Why it matters
Each duplicate appears once in the output?	Yes — the output

The solution is hidden so you can try first

6 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: findDuplicates() correctly identifies repeated values, returns an empty array for unique input, and preserves first-occurrence order of the duplicates

Practice

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