There apparently was a miscommunication along the lines somewhere that Gun Licenses only care about caliber, so if you own a 10mm gun and are licensed for it, you can buy any 10mm gun legally.
However, it is apparently coded differently than that. In order to buy a gun legally from the only legal gun source in the game, the gun has to match the exact caliber and serial number of that which was submitted with the license.
In short, Firearm Permits are for SPECIFIC firearms.
So try to remember that going forward. This could change if we decide to change it, but given that Corporations offer discounts on firearm permits, it is of my belief that we did that purposefully because it was believed that you had a reason to purchase a permit more than once.