Duplicate Stamps are a fact of life in Stamp Collecting.
I take the view that most of my duplicate British commemorative stamps from the 1990s are of absolutely no value to me. If they are of some use to another collector, then I will happily trade them for American commemorative stamps issued after 1998. On a one for one basis.
These British stamps are postally used and often have heavy postmarks. I am only rarely tempted to trade stamps. I am uncomfortable with the wording in some requests that I have seen on the Internet. People are too precise, too demanding, too obsessed with catalogue value, too selfish in wanting to (at least) break even on every trade. I just believe in friendly exchange. If forty of my stamps are useful to you and only ten of yours are useful to me…then I consider it all worthwhile. Things balance out. But I dont want to be ripped off.
The position with my Irish duplicates is different. I do have some mint and used but I DO consider that they have a “trading value”. Ireland is after all a small nation like say Luxembourg or Slovenia. It has a small number of collectors and therefore I think it is reasonable that I look on the duplicates as “currency” and I can hold out for exchanging for other Irish stamps that I actually want.