It's somewhat fixed. Higher tier parts are less likely to be repeated, Tier 1 parts, especially for those weapon categories where it's hard to unlock parts (2 handed axes are particularly terrible on that front) do that quite often.
Looking back, it seems to me that the basic, pre-unlocked parts of each class almost always "unlock" a second time, almost as if the game didn't count them in the blueprints initially. Pommels are particularly stubborn to go away, but it might just be that there are many similar ones at that tier.
