Execution of "merchant_topup" results in the return of the "SUSPECTED_DUPLICATE_ORDER" error. What could be causing this?

If the following conditions are satisfied when the current topup and the most recent topup transactions are compared for the same user, the "SUSPECTED_DUPLICATE_ORDER" error is returned.

1. The most recent topup amount and the current topup amount are the same, and
2. The most recent topup finished within 5 minutes before the current topup.