yozuul,
const clients = [
{ name: 'John', age: '25' },
{ name: 'John', age: '25' },
{ name: 'Tom', age: '30' },
{ name: 'Tom', age: '42' } ], temp = {}, sameClient = [];
clients.forEach(({name, age}) => {temp[[name, age]] = (temp[[name, age]] || 0) + 1;temp[[name, age]] == 2 && sameClient.push({name, age})})
console.table(sameClient)