Step 1: Ensure that your Brex transactions have receipts and memos associated with them prior to exporting; you can see which ones are missing by using the top filter bar on the Transactions tab.
Step 2: When you're ready to export, navigate to Integrations and click Manage next to QuickBooks.
Step 3: Click Review & Export.
Step 4: Click Export.
As long as you haven't yet closed your books on QuickBooks, QuickBooks will automatically match any transactions in the bank feed to data exported from Brex (receipts, memos, and categorization). You'll then have the option to confirm whether these matches are accurate by clicking Match to the right of each transaction on the bank feed. You shouldn't see any duplicates.
If you're still having issues, please reach out to Brex support via your dashboard.