Parse Server Cloud code keeps sending push notifications - push-notification

Question

I've developed an app that sends push notifications using Parse Server Cloud code. These notifications are received correctly in the devices but hours later they are automatically sent from Parse Server again (and they are received again). This happens 3 or 4 times for each push notifications.
If push notifications are sent from Parse Dashboard they are only sent once, so it seems it's a problem of my cloud code.
This is my code:
Parse.Cloud.define("sendPushNotification", function(request, response) {
var userId = request.params.userId;
var message = request.params.message;
var queryUser = new Parse.Query(Parse.User);
queryUser.equalTo('objectId', userId);
var query = new Parse.Query(Parse.Installation);
query.matchesQuery('user', queryUser);
Parse.Push.send({
where: query,
data: {
alert: message,
badge: 0,
sound: 'default'
}
}, {
success: function() {
console.log('##### PUSH OK');
response.success();
},
error: function(error) {
console.log('##### PUSH ERROR');
response.error('ERROR');
},
useMasterKey: true
});
});
I had a similar issue sending emails from another cloud code function (not included in the question) and my problem was because I forgot to add response.success(); and response.error('ERROR'); methods.
So this time I was sure to include these 2 calls in the responses of "sendPushNotification" method.
After sending a push notification the logs show this:
2017-07-09T15:38:02.427Z - Ran cloud function sendPushNotification for user undefined with:
Input: {"message":"This is my message","userId":"myUserIdInParse"}
Result: undefined
I think that this "Result: undefined" could be related with the problem because success and error functions are not called.
What could be the problem with this code? Why the code doesn't receive a success() when the notifications are received correctly in the devices?