Hi there,
Alex from the import/export team here. We just finished the swing on this feature. Now, you can create a property that requires unique property validation within property settings, and use this property as a unique ID within your import across contacts, companies, deals, tickets, and custom objects. These unique IDs can be used to find existing records within your CRM, and dedupe/update them with information in your import file.
Give it a try, and please let us know if you hvae any feedback!