Users can synchronize SharePoint Online calendar, contacts, and tasks with Microsoft Office Outlook and then view them offline in Outlook. But users of Outlook 2007 may encounter problems with duplicated items in SharePoint Online lists when they perform synchronization. A fix for Outlook 2007 is available to address list item duplication when synchronizing Outlook 2007 with SharePoint Online.
Organizations using Outlook 2007 with SharePoint synchronization must install an Outlook 2007 patch to prevent item duplication. Information about obtaining and installing the patch can be found in KB974994. Failure to install this patch may result in duplicated calendar items or duplicated contact items in SharePoint Online lists when synchronizing more than 700 items.
If you've questions contact Microsoft Online Services Technical Support.