"You will receive a notice 42-49 days before the end date of your fixed term tariff to inform you that your tariff is coming to an end. Should you choose to switch after this point, suppliers can’t charge you a termination fee."
For more informaiton on this see Ofgem's guideline to fairer treatment - www.ofgem.gov.uk/simpler-clearer-fairer/fairer-treatment
If you are in the last 42 days of your contract with your current supplier, you cannot be charged any exit fees and you will not be charged twice for the energy you use. Remember this is 42 days at the point of the switch, i.e. your supply start date, and not the date you sign up with us (as it does takes 21 days to get you on supply with us). If you're still unsure whether you'll be charged exit fees we recommend checking the terms of your current energy contract or speaking to your energy supplier.