I just had something weird happen today - 1st time I've EVER heard of this, and not even sure how it's happening...

I received 2 orders, both from the same person's account, but the billing address and credit card are different people. The customer who's account the order was placed under has no idea who the person is...

For example - order #1 is under the account of Penelope Smith - shipping & billing address & credit card are hers. She places her order, and everything is fine.
5 minutes later, order #2 is placed under the account of Penelope Smith. The billing address & credit card are under the name of Ethan Jones. The phone number & email address are under the name of Penelope Smith. Penelope emails me and says "What gives? Who is this Ethan person?"

Then about an hour later I get an email from someone completely different who says that he can't seem to finish checking out because the name on the account is someone totally different - his name is Jack Anderson, but the name on the account (when he goes to check out) is Lauren Peabody.

These people have no clue what's going on, so I know it's not a hijack, but at the same time, how can 2 people be logged in under 1 name when they don't know anything about the other? Is this a session problem? And how do I fix it ASAP?

Thanks!
Michelle