I exchanged a mutagen for 1 x Unicorn Festive Outfit Pattern and I just found one in a treasure chest while on exploring. However, when i check my inventory, i see that i only have 1 Unicorn Festive Outfit Pattern when i should have two, one from trading a mutagen and the other from finding it in a treasure chest.

Unfortunately, I didn't take a screenshot of finding the Festive Outfit while exploring, but i can take a screen shot of my mutagen and outfit trade.

I see that you have this pattern in you recipe library. Did you find / traded third copy of this recipe? Otherwise you just used it and it's in your library now.