I discovered just this morning that there are in fact, at least two different Underworld Trading 450 missions, which you can run at the same time. They are "Skimmers" and "The Brotherhood's Best". Despite granting totally different missions, they have exactly the same icon, and there is nothing in the description to indicate they are different. The only way to determine whether you have two different ones is to go through and try to use them all to see whether or not it allows you to learn a second one while one is currently active. This would be a ridiculously easy issue to fix, all that would need to be done would be to swap out of their icons to be different.