Get duplicates

dup-filter.js

const _contacts = [
    { Name: 'Andy', Emails: ['[email protected]', '[email protected]'] },
    { Name: 'Jason', Emails: ['[email protected]', '[email protected]'] },
    { Name: 'Bart', Emails: ['[email protected]'] },
    { Name: 'Richard', Emails: ['[email protected]'] }
];

const byEmail = {};

_contacts.forEach(con => {
	con.Emails.forEach(email => {
		byEmail[email] = byEmail[email] || [];
		byEmail[email].push(con);
    });

});

const dups = Object.keys(byEmail)
	.filter(email => byEmail[email].length > 1)
	.reduce((output, email) => {
		output[email] = byEmail[email];
		return output;
	}, {});

console.log(dups);