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The bank under fire for its Harriet Tubman debit card design says its sales are the only thing on fire ... 'cause new and old customers are clamoring for the new debit card.

We're told OneUnited Bank -- claiming to be America's largest black-owned bank -- is seeing exponentially larger volume than usual after debuting the Harriet Tubman card. We're told the bank is seeing 10x their normal volume in terms of bank account sign-ups, account holders requesting to switch to the new debit card design and banking inquiries.

It's only been 72 hours ... but OneUnited knows when they've got a huge winner 'cause they saw similar numbers back in 2016 after partnering with Killer Mike's #BankBlack movement.

The goal back then? Move money to black-owned banks. OneUnited claims more than $50 million in deposits was moved to black-owned banks across the country. This time around, OneUnited feels it's got a hit on its hands based on the early numbers.