Suppose a medical service is provided on December 30, 2015 and the bill is paid on the same day. The provider accidentally re-bills for the service in January 2016, and the bill is paid again. Shortly after realizing you paid for the same service twice, you contact the provider and they issue you a refund. Now you have receipts in two different tax years. Obviously you can't deduct them both in different years, so which year should you deduct the expense?
I suspect the reasonable answer would be you should never have a choice, as the situation must dictate the actual tax year. But in order for that to be true, then it must be clear which payment the refund was applied to. Normally it seems logical that if both payments and the refund were for the same amount, then the refund would undo the second payment, but is there an actual rule that would determine this? For example, suppose the 2nd charge was slightly more than the 1st, and the refund was for the 1st amount. Does this undo the first payment meaning the deduction must be taken in 2016?