They credit the account only.

Even if they would give you a check back, by the time you get it sorted out the next bill will have already issued and you wont have a credit anymore.

I once double paid by accident and was only given a credit

Nope. Happened to me too (twice). Gave me a credit, that's it. Next month you will have a light bill.

Watch the e-mails. Pay once and make note of it. Ignore the duplicate requests for payment.

They give zero shits

If you did electronic payment then I would see if you can through your method of payment to get it back. If a credit card, call them and tell them you paid twice, if debit contact your bank. That is your money and no where does it say they are entitled to keep your money.