This issue arises when Access Pay & Bill tries to delete the import csv file while new documents are being imported and, because the original import file is still open, the original can't be deleted and already imported records duplicate and cause errors.
To fix this, you can delete the errored records as follows:
Make a backup of the Documents Import File (for safety).
Delete the errored lines from the original csv (usually near the start of the file if it's a duplication issue).
Once you've done this, attempt to run the import again.