Outlook 2016 updating conflicts
If this is a frequent problem, you might want to avoid enabling New Event notifications for any calendar other than your primary calendar.
Also make sure you're using the latest release of Google Apps Sync with the latest hot fixes. A message that has multiple labels in Gmail appears in each corresponding folder in Outlook.It therefore looks like there are multiple copies of the message in Outlook, and the size of your local mailbox (PST file) can be larger as a result.To do this, right-click the Google Apps Sync icon in your Windows system tray, open the Synchronization Status dialog, and choose the Delete and re-sync option for your calendar data, only.
(Note that doing this will delete Outlook-specific data from your calendar, such as colored categories you've applied to events.) For details, see re-sync or repair Outlook data.If it does, the sender needs to modify their Outlook settings to use i Cal, as follows: Note that this setting might be disabled if you're using Google Apps Sync or if you're connected to an Exchange Server.