Greetings,
To exclude duplicates on load you need to set the property in IAddRasterParameters:
IAddRastersParameters AddRastersArgs = new AddRastersParametersClass();
... setup crawler etc...
// Exclude duplicates
AddRastersArgs.DuplicateItemsAction = esriDuplicateItemsAction.esriDuplicateItemsExclude;
Hope this helps.
Robert