I had duck eggs hatch out on three separate days from the same setting. So it's not unusual. However, the mama may get impatient and leave the last one. I have never had a broody, so I don't know how long she's likely to wait. I would be ready, though, in case she does. Do you have an incubator? Can you borrow one and get it set up for hatching if necessary?
If not, there are a number of things you can do that *might* be successful in helping the little one to hatch. Once it is hatched, you can tuck it back under mama at night, and it should be fine.
If you don't have an incubator, post again and hopefully someone will post ideas for getting that last one to hatch. I know it is sometimes successfully done in a bra (yes, while the bra is being worn, lol), but I don't know that that is likely to be the most successful way.