Charging twice

Bulb energy and British gas keep charging me twice in one month without a reason can you please explain this to me because I though only one of them will take money

I assume you have switched to Bulb. BG will continue taking your DD until they close your account which can take up to 6 weeks,
There can be a bit of an overlap but you will not pay for the same energy twice. BG will credit back any over payments when the switch is complete.