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 their SharePoint Online lists when they perform a synchronization.  A fix for Outlook 2007 is available to address list item duplication when synchronizing Outlook 2007 with SharePoint Online. 

Steps Organization using Outlook 2007 Must Take
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 Microsoft Knowledgebase Article 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.

The Microsoft Online Services team is committed to providing the highest quality service experience to our customers. Organizations that have questions should not hesitate to contact Microsoft Online Services Technical Support.