Great question! This is an issue we deal with on our farm. Sometimes corn kernels that dropped before harvest or managed to escape the combine during harvest will sprout the following season. These seeds show up as volunteer or rogue plants in our soybean fields. Not all of our corn is glyphosate tolerant so this is not a concern in some fields. On our farm all of our soybeans are Roundup Ready and obviously any small amount of Round Ready corn left over from the previous season that sprouts won't be killed by a glyphosate application. One solution is to spray the corn with something else. There are several herbicides labeled for soybeans that will kill volunteer corn.
Not all herbicides kill in the same fashion. The different methods of herbicide control are called modes of action. So all you need to do to kill off rogue corn plants in a soybean field where both plants are RR is use a herbicide other than glyphosate labeled for soybeans that will kill a corn plant. This method adds about $5/acre to spraying Roundup alone.
The other method is one we still use somewhat regularly. In fact I was just doing it earlier this month. We simply walk the bean fields with a hand held weed hooks and cut the corn down. Much of the soybeans we raise are for seed production so purity of product is very important. In our seed fields we can't afford to have rogue corn plants setting seed that might end up in our soybeans. In our other soybean fields we might not mess with the rogue corn at all if there isn't very much present. We generally are on a 50/50 corn and soybean rotation, but there might be an odd time when we have popcorn following regular corn. That circumstance requires hand weeding if we get many volunteers. I hope that answers your question!