I want to request a plus-one to a wedding
Why do you need another wedding invitation?
I want to bring my single friend with me.
I can understand that you would like to take your friend to the wedding, but asking for another invite intrudes upon the bride and groom's plans. Wait until the next wedding to take your friend.
F*** you guys.
Well you can ask but if it is really close to the wedding date you should hold off for now. If it is a few months out they will have time to order extra food and get the seating right.