Nodejs function fired twice

I have the following function, and it always fires sendQuery(...) exactly two times but it should only once. Therefore the second action causes a rude internal server error.

DatabaseController.prototype.checkLogin = function(appData, data, callback) {
  console.log('Check login...');
  var queryData                 = { /*parameters*/ };
  var cnt = 0;
  sendQuery(appData, queryData, function(serverRes){
    console.log('Send login query counter: ',cnt);
    cnt++;
    if(serverRes!=-1)
    {
     if(serverRes.succ == 1)
     {
       console.log('callback: serverRes');
       callback(serverRes.dbData);
     } else {
       console.log('callback: !succ -1');
       callback(-1);
     }
   } else {
     console.log('callback: !serverrres -1');
     callback(-1);
   }
  });
}

The output/console log looks like:

Check login...
Send login query counter:  0
callback: serverRes
Send login query counter:  1
callback: !serverrres -1

This issue is really strange, because as you can see the checkLogin() just fires only once. What can cause this?