add 'already activated' response

src/auth/operations/verifyEmail.js

@@ -10,15 +10,14 @@ async function verifyEmail(args) {
   // 2. Perform password reset
   // /////////////////////////////////////
 
-  // TODO: How do we know which collection this is?
   const user = await args.collection.Model.findOne({
     _verificationToken: args.token,
   });
 
   if (!user) throw new APIError('User not found.', httpStatus.BAD_REQUEST);
+  if (user && user._verified === true) throw new APIError('Already activated', httpStatus.ACCEPTED);
 
   user._verified = true;
-  user._verificationToken = null;
 
   await user.save();
 }