After downloading the IIF file and importing it into QuickBooks, instead of importing the transactions into the current account it creates a brand new bank account.
Most Financial Institutions (FI) offer downloads either by Direct Connect (download directly into QuickBooks) or by Web Connect (download and import QBO files). Intuit suggests using one of these methods instead of using IIF files. Please be aware that Intuit does not provide live phone support for IIF issues.
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Import the IIF file and merge the new bank account into the existing bank account.