If you get multiple entries, normally, only the last one should be confirmed and the others cancelled.
That's because before you go to authorize.net for the payment, the order has to be created in HikaShop. Then, when the payment is done, the order is confirmed. If however the user clicks on cancel in authorize.net, his order is cancelled and he goes back to the checkout. He can then again go to authorize.net for the payment, etc. That way, you can end up with several cancelled orders but that doesn't mean that there is a problem. It's just the user which is going bakc and forth between your checkout and authorize.net's payment page.
No, they don't have to log in the backend to do it manually. That should be done automatically. If that's not the case, it means that there is potentially a problem with the payment notification. That's why I gave you a link to the FAQ on that. IT could also be that you configured Authorize.net to not capture instantly the payments and thus they are only authorized and need to be captured manually in the Authorize.net account. You should check the "instant capture" option of the payment plugin.