how to make it so there cant be duplicate displayNames in firestore code example

Example 1: how to edited the firestore from front-end

const updatePopulation = (city, pop) => {  cities.doc(city).update({    pop: pop  })}

Example 2: how to edited the firestore from front-end

const b = document.createElement('button');b.textContent = '+100';b.onclick = () => updatePopulation(city.name, city.pop + 100);p.appendChild(b);