From time to time users have issues with Publishing, UI does not receive event that the page was published. SO they Publish the page again, and again.
But actually page was published fine, just the notification did not get trough to user.

In the logs I see a lot of errors:

System.Web.HttpException (0x80004005): In use notification already exists.
   at EPiServer.Cms.Shell.UI.Rest.InUseNotificationStore.Post(InUseNotificationViewModel entity)

Is this is a Websocket issue with firewall?