Symptoms
Consider the following scenario:
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User A is a Microsoft Skype user. User B is a Skype for Business Server 2015 on-premise user.
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User A adds User B to the contact list.
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User B accepts the notification to add User A to the on-premise contact list.
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User B deletes User A from the on-premise contact list. User A deletes User B from the contact list in Skype.
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User A adds User B to the contact list again.
In this situation, User B doesn't receive the notification about the contact request from User A.
Resolution
To fix this issue, install the July 2018 cumulative update 6.0.9319.534 for Skype for Business Server 2015, Core Components.
Workaround
To work around this issue, the Skype for Business Server 2015 on-premises user can add the Skype user to the contact list.
