Hi @Erictrinh the previous supplier should only be billing you for usage before the switch date. Sometimes this can mean an additional payment up to the meter readings used on the switch. If so, this should be outlined in a “final bill” from the previous supplier.
If you believe despite this there has been an erroneous charge, would you be able to let us know the dates and property this is in relation to at firstname.lastname@example.org?
If in doubt you can always cancel the direct debit from your end whilst it’s sorted out.