Cannot equip two Durmand's Pen
This poor little man. He spent so much gold for nothing!? He deserves a refund! GAHHHH!
I completely disagree with anything you say. If ArenaNet were to allow you to equip 2 of the same ascended, it would be extremely overpowered and for once people would actually be good. We all know them, they have to have at least one laugh at the players (IE: Madragor Mass Ressurect in Guild Wars 1)
You cannot have more than one unique. That is how the system works, though they really need to put some sort of emphasis on that. Try putting a different infusion in it. I hear that works for some people.
You can, if you so desire, contact CS and they will, usually, exchange the item for you. Good luck.
You can only have one of each ascended trinket equipped at a time. Adding an infusion to the infusion slot won’t change the internal ID of the item. However, if you want to upgrade the item to its infused version, it will become a new item. e.g. you can have Durmond’s Pen + Durmond’s Pen (infused), but not two of each.
As the ever-helpful (and 99.9999% correct) inculpatus cedo suggested, create a support a ticket and support will help you out.
Good luck.
Just for reference, there is another PVT earring called Golden Relic of Rin (should be right next to Durman’s Pen in the commendation trader). This is what you will want to get instead of a second Durmand’s Pen.