I’ve received 2 of these during my playthrough most likely because I did not add the 1st one to my skills immediately.
The game itself says it treats the runes of the same rarity as unique: does not allow more than 1 instance of each kind, and does not drop the 2nd rune of the same kind, or at least it worked that way with any other rune.
I could not drop the duplicate Rare Galewind, nor sell. The only option available was to “consume” the rune so it won’t take space in my storage - unfortunately this created a 2nd instance of Rare Galewind. (I could have however created a new character, pass it on to him and then delete. Sadly, it occurred to me much later after I already added the 2nd instance of the rune to my skills.)
Please fix. Having 2 8% Galewinds is not intended, feels like cheating, and is giving me a slight OCD…