Prevent validate and observer methods from being triggered twice

Steps to observe this behavior:

1. edit nxserver/nuxeo.war/ui/document/file/nuxeo-file-edit-layout.html and add these lines

       observers: [
         '_titleChanged(document.properties.dc:title)',
       ],
       
       _titleChanged: function() {
         if(this.document) {
           alert("dc:title has changed");
         }
       },
   
       validate: function() {
         alert("Validation is done");
         return true;
       },
   

1. force reload the Web UI
2. navigate to a File document and open the Edit form
3. change the title, observe the alert
4. submit the Edit form for a File document
5. Observe that the validation alert is displayed twice and the observer alert is also displayed twice

Even if these methods should be idempotent, this double trigger could bring to a global performance issue when a form has many observers