Correct adding new users handle (#4055)

If user has already existed, response error should be 400.

Author: minhtaile2712

locale/en/docs/guides/simple-profiling.md

@@ -37,7 +37,7 @@ app.get('/newUser', (req, res) => {
 
   username = username.replace(/[!@#$%^&*]/g, '');
 
-  if (!username || !password || !users[username]) {
+  if (!username || !password || users[username]) {
     return res.sendStatus(400);
   }
 
@@ -59,7 +59,7 @@ app.get('/auth', (req, res) => {
 
   username = username.replace(/[!@#$%^&*]/g, '');
 
-  if (!username || !password || !users[username]) {
+  if (!username || !password || users[username]) {
     return res.sendStatus(400);
   }