A "DT" decline code means that you have already charged the same credit card, for the same amount, with the same invoice number within the last 10 minutes. It's essentially a duplicate transaction.
As a safety precaution, PayJunction automatically flags any dup transaction and declines it with a DT response code. This feature has prevented many accidental charges for our merchants.
If you actually intend to charge two transactions to the same card in the same amount, you may bypass this duplicate transaction security check by waiting 10 minutes to try again, or by specifying a different invoice number to immediately try again.