Symptoms

When editing a Word (or Office) document online, it offers to combine the documents when it has only been edited by one person: 

Resolution 

In Word 2016 configuration

File > Options > Trust Center

Click on the “Trust  Center Settings”  button 

In the new window that appears, click on the “File Blocking Settings”  menu

Check the last radio button instead of the 2nd.

Uncheck the first box in the “Protected  Mode”  tab: