Posts are duplicated

Hello!
When I export posts from a custom post type, it exports indeed file wpcsv-export.csv.
If I add or edit some entries, than import that file, it updates successfully the posts.
But for some reason, I would like to edit only some entries, so I paste some rows to a new csv file (in order to upload a smaller file and spare time during import).
The problem is that it ADDS some posts and not edit them.

Is that a known bug? Is there any chance to correct this?

Regards.

https://ww.wp.xz.cn/plugins/wp-csv/