* They pretty much do by not giving a "new recipe unlocked" popup.
* They infact do...
Yeah well of course they don't give a recipe unlocked popup if you already know them but that's not my point. What I mean is that they should show something along the lines of "you already know this" when inspected at vendors. Skill books do that and I don't see any reason why recipes and maps shouldn't do the same. Having to exit the trade interface to check the relevent journal sections to see if you already own them or even having to buy and try them is about as clumsy as you can get there. Oh and regarding treasure maps, would be nice to see which spot is from which treasure map on your map, they all just say "mound".