One of our customers was charged 10 times via Authorize but in Magento there is only 1 order, 1 invoice, and 1 transaction. So she was charged 9 additional times. All but three of them have different invoice numbers. The other three share the same number and none of these numbers can be found under the admin panel. What scenario could even cause something like this to happen?
Please try checking the Authorize.net log file in var/log folder. It will tell you the number of times a payment request has made by your customer.
If there is only one entry for that particular transaction, then there is nothing wrong the transaction. You might wanna contact Authorize.net to sort out this issue.
Just a quick question though, have all those transactions been approved? Have you matched the transaction number in your magento admin panel as well?
Have you kept your magento installation updated or at least security patched?
There are a number of known vulnerabilities some of which could potentially hijack your customer's checkout info and repurpose it into their own merchant account.
As this is a Magento forum, I'm guessing your know these things, but posting them anyway for folks who may just be getting here.