title: Creating increment limit
body: <p>Ok so I am making a like/dislike system for my website and I got it to work. The only problem is, is that a user can like something more than once, how do I stop this? heres my function</p>
  <pre><code>    function thumbup(uid, s) {
          const prevthumbs = s.thumbs
  
              firebase.db.collection('users').doc(uid).update({
                  thumbs: prevthumbs + 1
              })
              
      }
  </code></pre>
  <p>How can I limit the increment to only one?</p>