Paying already paid ln invoice doesn't fail and spends the fees #782
Description
If you try to melt tokens to pay already paid invoices you won't get any error. Instead new melt quote is created and when you execute it the quote just stays in the pending state until the invoice expires. When the invoice expires, the quote moves to paid state for some reason and the invoice amount is returned as change, while reserved ln fees are kept by the mint. For example, if invoice amount is 1 sat and ln fee reserve is 2 sats, you will get 1 sat back while mint will keep 2 sats.
I was expecting that one of the two melt requests will return some kind of error instead. If that for some reason can't be done, it should at least not take the fee for ln payment that is never made.
You can reproduce this by making 1 sat invoice in Phoenix or some other wallet and then trying to pay it twice with minibits, lnvoltz or any other mint that uses nutshell.