You'll get a 'duplicate transaction' error if after successfully sending a particular amount of money to a beneficiary, you attempt to send the same amount to the same beneficiary with the same transfer description or note within 10 minutes of making the previous transfer.

How do you avoid this?

If you need to send the same amount of money to the same beneficiary twice within 10 minutes, as in sending 2 million naira as two separate transfers of 1 million naira because you can't transfer more than 1 million naira at once, just change the description or note for the second transfer.