Yeah, you can only own one of some of these items, like the masks. If you "win" more than one, you get one, while the extras are converted to oddments.
Aslon Dak: Still 'Scaping after 19 years!
You can buy XP. You can't buy experience.
"...From The 'Scape" is very nearly here.
If you could get more than one of a single type of Slayer Mask, it would mean you don't have to prestige it so you can get the benefits again and again lol.
Like say you got 4 Mask of Abyss (which is unique and impossible to get more than one of but this is just hypothetical) it would mean you can get the extra exp and extra "Double Drops" while wearing each mask without using the reset item on it aka: prestige it so it's no longer a helm which wipes the kill count but enables other bonuses.
Idk if they can be keepsaked or not... never tried. But if you have a mask/helm you particularly like and if this is possible, I suppose getting more than one would work with that too
Though I doubt they can be keepsaked.
But again all that is just joking around as the masks/helms are unique and you can only have one at a given time. If you win multiples, you get 1 and the rest are converted.