#0308

Find the First Duplicate Element

EasyPython15 min10 XP

Problem

Given a list of hashable values, return the first value whose second occurrence appears earliest. If there is no duplicate, return None.

Why This Matters

Duplicate detection is one of the first useful set patterns: remember what you have already seen.

Function Signature

def first_duplicate(items):

Examples

Example 1

Inputitems = [1, 1, 2, 3]

Output1

The second 1 is the first repeated value encountered.

Constraints

Return the exact requested value.
Handle edge cases cleanly.
Prefer clear Python code before clever one-liners.

CodePython

def first_duplicate(items):
    """
    Input example: items = [1, 1, 2, 3]
    Return: 1
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return None

def first_duplicate(items):
    """
    Input example: items = [1, 1, 2, 3]
    Return: 1
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return None

Visible browser tests run here when available.

Testcases2 visible / 3 hidden categories

Immediate duplicate

Input[1, 1, 2, 3]

Expected1

1 appears again before any other value repeats.

Later duplicate

Input["a", "b", "c", "b", "a"]

Expected"b"

b repeats before a repeats.

Hidden Test Categories

no duplicatesempty listduplicate None value