Ethics versus practicality. Ethically, return the dress - which was probably sewn by exploited women who are paid a pittance, the vast majority of the profit going to the firm you ordered it from.
Practicality - stick it in a cupboard and if they don't come asking for it after a couple of months, flog it or wear it. If you are challenged about this at a later date, you may have to say that you tried phoning but nobody got back to you.
if she just informs them of the double order they may just lket her keep it, there are a surprising amount of companies that dont want to stump up for postage. depends on the cost of the item I suppose.
Tescos once cocked up on a DVD I brought, they charged me the full £25 (I think) as opposed to the £15 it actually was in the sale... I took it to customer services and they gave me a full refund and let me keep the DVD for free, apparently it was their policy that if they made a mistake then you don't pay for the item. That was in Guildford, I was particularly chuffed :c)
I had something like that with tesco, bought six bottles of wine and didn't get the 5% off, had only glanced at the bill, went straight to CS who looked at the bill in detail and said, that's cos only 2 bottles have been rung up. I pulled out my wallet to pay but they said no it was their error, my gain.
Google law and unsolicited goods...you will almost certainly find a letter template to send them....that may well end up with you keeping the item legally