I also stumbled over this, would be great to decide with an Option whether:
a) push notifications are only sent when the user is logged out everywhere (Status quo)
b) push notifications are sent to mobile even when user is currently logged in elsewhere
Probable Use Case: Stil logged in on Desktop in the Office, but being on the road with a smartdevice
I opened up a Feature Suggestion/Request 
https://mattermost.uservoice.com/forums/306457-general/suggestions/13834386-option-to-push-notify-to-mobile-even-when-logged-i