Allied Seals vendor bug

This bug allows for potentially unintended expenditure of Allied Seals, as well as the strange side-effect of being able to possess more than one of a Unique item in the player's inventory at the same time.

Time: 9/30/2016 6:30 AM PDT
Frequency: Anytime
World name: Hyperion
Character name: Fynlar Eira
Retainer name: N/A
NPC name: Hunt Billmaster
Monster name: N/A
Class/Level: N/A
Party or solo: Any
In-game time: Any
Area and coordinates: New Gridania x9 y11
Housing: N/A
Steps:
1. Be of the appropriate Grand Company for respective Hunt Billmaster NPC (I am assuming this bug works for both of the other Grand Company vendors)

2. Talk to Hunt Billmaster, select "Allied Seals (respective Grand Company)"

3. Select the "Others" tab

4. Change the buy quantity of any item in the list. (Example: change the quantity of the first item, Unstained Mark Log, to 2)

5. Now select the "Weapons" or "Armor" tab without closing the window.

6. The piece of equipment corresponding to the item slot for which you adjusted the quantity will also have its quantity adjusted, with the Allied Seals price adjusting appropriately -- even though you are not supposed to be able to possess more than one of the equipment at a time due to being Unique, and even though the quantity is not actually shown until attempting to purchase the equipment. (Example: after changing the buy quantity of Unstained Mark Log to 2, selecting the "Weapons" tab will now show its first item's price, Serpent Elite's Longsword, doubled to 3480 seals from the usual cost of 1740, as if you were trying to buy two of the item.)

7. The game will allow the player to purchase the increased quantity of weapon/armor as normal, if the player possesses enough seals. (Example: if the player actually has at least 3480 seals in their possession, they will be allowed to make the aforementioned purchase. They will end up with two Serpent Elite's Longswords in their inventory, and will have expended 3480 of their seals.)

As an addendum, I have found that this same bug is present with the Scrip Exchange NPCs in Mor Dhona and Idyllshire, when attempting to purchase items using blue scrips, in a similar manner.

Indeed, it appears to happen with any of the vendors that sell both unique & multiple items. I think the recent bug report on Lodestone is about this but it's rather difficult to decipher.