Why is there no CSV import option for Fiverr?
Because Fiverr does not offer a unified transaction CSV export. Order invoices are per-order PDFs. Tally Assistant reads those PDFs and earnings screenshots instead, which covers the same record keeping without a format Fiverr does not provide.
How do I avoid double counting Fiverr income?
The withdrawal from Fiverr to your bank appears in both records. Categorize the withdrawal rows in your bank import as transfers. The income itself is recorded once, from the order invoice or screenshot.