Have you tested your powder/charge to see what a double looks like? This is one of the reasons I got started reloading using 124gr 9mm and Universal Clays. A double charge will overflow the case. Even with the smaller charges suitable for 147gr, a double of Universal will pretty much fill the case such that it'd be hard not to notice. I also agree with a previous poster, frequent interruptions (to weigh powder charges or for any other reason) are a great way to introduce human error and end up with a double charge or squib. The squibs I've had were, AFAIK, all due to stopping to deal with primer slide malfunctions, and screwing up when getting back to loading.