Old Supplier
When you leave a contract with any supplier, they will use your final meter reading to calculate how much energy you used in your time with them, and share what is owed with you in a final bill. This will be either a refund (if you've overpaid and have built up credit), or a final charge (if you've used energy that you've yet to pay for).
As this final bill can come up to 6 weeks after your switch has completed, you may find that you're paying both suppliers in the same month, which may feel like "double billing", but is actually just the settling of balances.
New Supplier
Your new supplier will typically request an upfront first payment for your Direct Debit, usually a few days after your switch is complete. This helps to maintain a healthy balance in your account. Over time, if you consistently make payments on time, you may be eligible for a refund from your supplier. To qualify, you'll likely need to have at least one month's payment and provided a meter reading. You can request a refund by getting in touch with your supplier.