Double billing was the least of my worries some years ago... ;-)
Years ago I was using GoDaddy. I got some shared hosting account with 100 gigs space (USD 75 a year). Of course I didn't know this space is for web hosting only, and not for backup purposes. So after one year or more I got an email telling me of my wrong-doing, I've to erase the files, else they'll be erased... bla bla bla... so there, my fault, no harm done. I erased my backup files and since I didn't need the 100 gigs account I downgraded to some smaller account, with 5 gigs....
And now the fun started. After the downgrade their system registered me with "hosting space overdrawn", because their "space used" calculation is following another cycle. So I got automatically billed hundreds of "Additional 100 MB" packages, which made about 7,000 USD. I found out about it, because I got an email telling me they tried to charge the money from my credit card, but it got rejected. I've been in a lot of luck, because it exceeded my limit only by a couple of hundred euros.
Anyway, no matter what I tried, the support insisted in paying the bill. After a while, I just gave up. They eventually suspended my account. I thought they'd send some lawyer after me, but perhaps someone figured out their mess.
