Assume that you send continuous instant messages (IMs) to a user in Microsoft Skype for Business 2015 (Lync 2013). Then you allow the toast notification window to be auto accepted. In the conversation window, you find every item but first gets duplicated. Also, in the Conversation History in Microsoft Outlook, you find that the conversation window shows duplicated IMs.
To fix this issue, install the January 3, 2017, update (KB3141468) for Skype for Business.
Note Lync 2013 was upgraded to Skype for Business in April 2015.