I imported 10 tasks with custom fields into a new project. Then later I tried to import 10 additional tasks to the same project. But instead of populating the existing fields, Asana created duplicates of some of the custom fields. for example, one of the custom fields was a text field for sub-account number. When I imported the second group of tasks, Asana left that field blank, and created another sub-account number field with the data. It did the same with a column called Notes.
The second import also filled in randomly among the existing tasks.
After 2nd import: