After importing a catalog via the Service API an issue can arise where duplicate items show in search results.
When creating the catalog.xml export file check that any GUIDs for existing items are not different from the catalog items in the CMS.
Example GUID in catalog.xml
If the GUID is not identical to the to-be-updated item, indexing will create a duplicate in search results.
It is often not necessary to add <Guid>....</Guid> so unless there is a specific requirement it should be removed.