Taking Martin Lewis’s advice, I duly switched suppliers only to be informed by the new supplier that they have checked their records and it appears that I am already a customer and owe them 3 years unpaid gas and electricity! 
This is absolutely not possible as I have been with another company for the past 4 years and have been paying by DD all that time.
New supplier say we owe nearly £4000! And that we must have “slipped through the net somehow”.
How is this possible? I’m slightly panicking as they have since sent a scary letter saying if we don’t pay, they’ll send in debt collectors! 
I’ve looked it up online and it appears that energy suppliers cannot ask for more than 2 years supply if reasonable attempts have been made by the customer to alert the company of unpaid bills.
Obviously I haven’t done this as I have been paying my bills, just not to this company!
How is it possible to have 2 live accounts?
If it makes a difference, they are both one of the big five.