Consider the following scenario:
You have enabled Busy Options feature for internal Skype for Business users in a Microsoft Skype for Business Server 2015 environment.
You have the following restriction in your manifest file:
You are in a call with another federated user.
A user is trying to call you.
In this scenario, you find double toast notifications.
This issue occurs because calls to federated users are not tracked by Busy Options when the domainSupported parameter is set to true.
To fix this issue, install the May 2017 cumulative update 6.0.9319.281 for Skype for Business Server 2015, Front End server and Edge server.