A lot of people mailed us about this. In short, reports have came in of Mythic's billing system going mental and charging people multiple times for their subscription. Go to the forum to see some of the reports, with some people reporting hundreds of dollars being vacuumed out of their bank accounts. The official advice at the moment is that the vendour will reverse payments in 24-36 hours, and if it hasn't by then, you should hit up your bank, who should reverse the charges (And if they don't, you should phone Mythic Billing). They also promise more news on the issue next week which - considering some of the tales of over-draft annihilating woe in the comments threads - better include at least some conciliatory compensation to those affected.
While this seems to be a US issue, I suspect anyone with a Warhammer account probably should check their account.