Can you salvage a plan/pattern?
Sadly, no, you can't. Plans are unsalvageable. What you did is the only thing you can do with duplicate plans. Or just drop it on the ground.
Trying to check Blizzard's site, plans don't even have their own page. You can see the list of plans, but clicking on one just brings you to the crafted item.
No you can't. I always return to town immediately to teach my blacksmith when I get a plan to avoid this situation occurring.