ignore existing subscribers on import

Is it possible to ignore previous subscribers on import. 

In our setup users can unsubscribe from email lists. This will leave them in the database without any email list assigned to them.

Our client wants to handle subscribers in an external excel sheet. They want to import the subscriber list now and so often.

The problem is that when importing subscribers the plugin will re-assign previous unsubscribers to lists instead of throwing an error that they are duplicates. That would mean that all unsubscribers would be reset.

Is it possible to ignore subscribers that are not assigned to any list on import?