Extra Property Added to Events: Identifying Users in SPA Applications

I'm getting extra property added to my events: $identity_failure_reason: erranondistinctidassignedalready For context I'm setting/invoking and identifying user upon login and on app init/page load for SPA routes as such:

// Identify upon login  
mixpanel.identify(<USER_ID>);
        mixpanel.people.set({
          $email: <USER_EMAIL>,
          $username: <USER_USERNAME>,
        }); 

// Example event afterwards during user being logged in
mixpanel.track('button_clicked', {
        button_name: 'submit_feedback',
      });

Am I missing something or doing something incorrectly? Or should I just identify user in once throughout application?