The Original Keeper

An easy Python interview practice problem on DataDriven. Write the Python and run it against test cases, with instant feedback.

Domain
Python
Difficulty
easy
Seniority
L3
Asked in
Amazon

Problem

Given a list of items (possibly with duplicates), return a new list containing only the first occurrence of each value, preserving input order.

Clean up duplicate events without losing the timeline.

Example

Input:

items = [1,1,3,2,5,6,5]

Output:

[1,3,2,5,6]

Worked solution and explanation

Why this problem exists in real interviews

This tests order-preserving deduplication, a fundamental ETL operation. It checks whether candidates use a set for O(1) lookups while maintaining the original insertion order in the output.

Break down the requirements

Step 1: Track seen values in a set

Use a set for O(1) membership checks.

Step 2: Iterate in order, keeping first occurrences

For each element, if it has not been seen, add it to both the result list and the seen set.

Step 3: Skip duplicates

If the value is already in the seen set, skip it.

The solution

Set-tracked first-occurrence dedup
def deduplicate(items):
    seen = set()
    result = []
    for item in items:
        if item not in seen:
            seen.add(item)
            result.append(item)
    return result

Common follow-up questions