We recommend to maintain the userName as unique while registering the user for Push Notification. Whenver you call storeDeviceToken API to register a user, server works as follows:
1. if user already exists and if the token received along with current request is same as the previuos token then you will get User Already Exists exception
2. if user already exists and if the token received along with current request is different than the previuos token then it will consider it different device for the same user and create one more entry for that user
3. if user does not exist but the device token received is already registered for another user name then server replaces the existing user name with the user name came with current request
As of now, get API for fetching the users registered for push is not available.
Please let me know if you have any further queries, I will be happy to help you.