Either a Corn double clutches or she doesn't. There's no way of preventing a second clutch as they can just lay slugs, or retain sperm from earlier matings and lay fertile ones.
As Erika says, now that you know you have one that is likely to double clutch, all you can do is ensure that she's in tip top condition for her first clutch, so that you can get her in as good shape as possible for the potential second one. After she's laid the first clutch, I'd advise getting as much food into her as she'll take - especially if she goes off her food for any length of time before laying. Mine usually eat one large mouse a fortnight, but the double-clutchers will get two mice at once after the first clutch. Also, I start to offer smaller food as they swell, to try and keep them eating as long as possible before the second pre-lay fast.
As you've seen, second clutches are reasonably easy to predict, usually being laid around the same time as the first clutch is hatching.
Laying a second clutch of infertile eggs isn't an inherently bad thing (as long as the female is in good shape to start with). Slugs are usually smaller and easier to lay, so if you don't actually *want* the second clutch, this option is less stressful on the female.
