At Outfox the Market you pay your first direct debit on the switch date. Some other suppliers take their payments in arrears so around the time of the switch you may see two payments to two separate energy companies go out. One will be for the energy you are yet to use and one will be for energy you may already have used.
The good news is you will never pay for the same energy twice as both suppliers use the same readings to open and close your account.
Your previous supplier will always refund any over payments and you should expect your final bill within 6 weeks of the switch date.