Most importantly, your seller had to have realized she was paid twice. I accept payments through Paypal and given that they keep a seller using their service updated on payments, she must've been contacted twice, particularly since you said the second payment went in some time (a whole day) after the original payment. I've had a few people click that Send Money button more than once and it always shows up on my end and it takes 2 seconds to cancel the transaction.
I would give her a deadline for which to refund your overpayment, regardless or face the consequences with eBay and Paypal. That is theft, plain and simple. Who gives a flying you-know-what if she needs to finalize more auctions to pay you? She shouldn't be spending money that isn't hers in the first place! She was probably hoping you wouldn't notice and was taking advantage of her little windfall.
Keep all documentation and be prepared to file your case with Paypal. Go to the Help section on their website and find the Buyer Complaint section there. Usually this pertains to sellers who don't deliver goods to buyers after receiving payment but maybe it help you recover your money.
Good luck and keep us posted!