Well, I think this would be a very easy to implement add-on. Sadly, don't know how to code add-ons myself or I'd give it a shot
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Anyways, this would just be a quick and simple tooltip modification, when hovering over something in your recipe book. The concept is simple, we get Bonus XP for crafting things we haven't made before - so why not optimize on that.
So, somewhere in the tooltip (below the recipes name for example), it will say something to the effect of "
Previously Crafted" or "
Never crafted before"... colors for those two wouldn't hurt either
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Anyways, I'm pretty sure that SOE doesn't forward along the items that a player has already crafted, so this would be something that the mod would track itself using an external file or some such as a log.