How to Find Duplicate Items in a JavaScript Array

findDuplicates.js

// 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) {
			duplicates.push(value);
		}
	});
	return duplicates;
}

const duplicatesFound = findDuplicates(arrayWithDuplicates); 

duplicatesFound.forEach( (duplicate) => {
	console.log("Duplicate: " + duplicate);
});