You should restart.Could i uncheck that option at this moment and continue with the import? or may i have to restart the import?
An alternative might be to manually remove the offending thread/post/whatever from the database:
But this is a dirty thing to do.
The actual option to preserve the source ids is stored in the import session. The import session is stored in the xf_data_registry table (data_key = importSession). You can run this query to view the information:
SELECT CONVERT(data_value USING utf8) FROM xf_data_registry WHERE data_key = 'importSession';
I recommend starting over.