Before proceeding it is highly recommended that you make a backup of your Outlook data and confirm that your Outlook data does not contain any duplicate calendar events, contacts, notes or tasks depending on what you plan to sync.
During the sync process gSyncit will attempt to match existing items between Google and Outlook. If a match is not possible duplicates may appear and require some manual intervention to remove possible duplicates. Before adding any items to Outlook or Google gSyncit will check for an existing entries based on predefined criteria
The duplication logic will pair existing items based on the item title/name and either the start date, due date, or email address + company name depending on the type of item you are trying to sync.