How to export it only for one custom field added many times for one product?
It is not possible for exporting many columns for one custom field and to get multiple values.
The reason behind this is that on import RO CSVI looks for custom field based on the name and when it finds it in database, it updates the existing value. So even if you have many columns in import file, RO CSVI will update the same field. To overcome this problem we have custom_title and custom_value so RO CSVI will insert the custom field as many times as given in these fields.