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How to Find Duplicate Items in a JavaScript Array
// Duplicates: Bob, Aaron, Frank
const arrayWithDuplicates = ["Aaron", "Bob", "Chris", "Dave", "Edward", "Bob", "Aaron", "Frank", "George", "Frank", "Henry", "Bob", "Bob", "Aaron", "Aaron", "Frank"];
function findDuplicates(arrayWithDuplicates) {
let duplicates = [];
arrayWithDuplicates.forEach( (value, index) => {
const indexWhereValueFirstAppears = arrayWithDuplicates.findIndex( (value_) => {
return value_ === value;
const indexWhereValueLastAppears = arrayWithDuplicates.lastIndexOf(value);
if (index !== indexWhereValueFirstAppears && index === indexWhereValueLastAppears) {
return duplicates;
const duplicatesFound = findDuplicates(arrayWithDuplicates);
duplicatesFound.forEach( (duplicate) => {
console.log("Duplicate: " + duplicate);
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