title: Check if Field Already exists in Flutter Firestore
body: <p>I have a collection called <strong><code>company</code></strong>.
  All the companies are going to be stored like in my screenshot.</p>
  
  <p>When I add another company, I want to check if the <strong><code>name</code></strong> already exists or not.
  How to perform that?</p>
  
  <p>Here, "Nova" and "Tradetech" are two companies.</p>
  
  <p>When I try to add <em>"Nova"</em> with the field <code>name: "nova"</code> again, I want to show a notice: <em>"Company already exists!"</em>.</p>
  
  <p><a href="https://i.stack.imgur.com/28iiI.png" rel="noreferrer"><img src="https://i.stack.imgur.com/28iiI.png" alt="screenshot"></a></p>