Was looking at Runesmith and realized that I'm unsure if I'm wrong or if playtest runesmith is wrong.
To my reading, the duplicate effects rule implies you only get the benefit of something once, not that only a single instance of a spell/rune/etc. can be exist on the target at once. There is nothing that says the instances of mystic armor replace one another, just that you cannot receive the benefits and only one of the effects can "apply"—where "apply" is ambiguous between "stick" and "receive the benefit of." That the opening clause of the passage says "when you're affected by the same thing multiple times" instead of "when you would be affected" implies it's possible, as well—especially given that the subjunctive is used not two sentences later in an example, making the choice to use an indicative conditional there look pretty intentional. And there are common sense cases—like equipping and investing two rings of fire resist—where there are clearly two instances of the effect, but you only gain the benefits of one. It's not like one ring stops existing when you invest the other.
I believe the rule only prevents you from gaining identical benefits twice, and does not prevent you from having additional and useless instances of effects and effect-givers, because of these reasons. But I do think the intent is less than clear as it's written.
The difference usually won't matter, yeah. It's primarily a meaningful difference for the purposes of dispelling. I.E., How many instances of mystic armor are there to dispel if it's cast on someone twice—one, or two? There are certainly cases where you can get one dispel off and have a second, identical effect replace the first—the Resist Energy spell and a ring that gives fire resist won't stack, but dispelling Resist Energy wouldn't get rid of the fire resistance from the ring, and the aforementioned case with two fire resist rings exists as well. So it isn't all THAT nonsense to have "backup effects." It would be odd—though not particularly gamebreaking, and pointlessly costly in slots or scrolls—to have multiple backup casts of mystic armor on you, sure. But I think it seems odd because no one does it, and no one does it since it's utterly wasteful. After all, having multiple casts of high rank mystic armor would take a ton of daily resources for no obvious benefit in the vast majority of situations. To me, this sounds less like it's broken and more like a fringe edge case where you waste resources to mess with people using dispel magic; most spells and effects don't last long enough for this to even be viable, anyways. If there even is a problem, it's a self-correcting problem.
So, running this back to Runesmith:
Runesmith is one of the only other reasons to care, since runes have both passive and invoke effects, and being able to have the same rune active on a target twice would let you invoke while the passive effect is also up. To me, it seems like it could be intended and even fine that—for example—placing multiple fire runes on an enemy and only ever detonating the oldest could give you the fire resist reduction on the detonation damage. The resist reduction scaling isn't even that significant compared to the damage scaling on the invoke. I could be missing something, mind. But given that you can only get the invoke benefit of a given rune type on a given creature once per invoke, I don't really see how placing multiple instances of a rune on the same target is a problem. Even something like whetstone can't affect the same target more than once per invoke. Even if it's invoked from etchings on two different weapons, the target will only take damage once because of the duplicate effects rule. And it's not like you can't already prestack a bunch of whetstone runes on a single person with the class as-written. (Everyone has an unarmed strike, and it doesn't say you have to be wielding the weapon to invoke the rune, so there's some real silliness here.)
However, the sun- diacritic implies you should only be able to have a rune type active once per target, because there's very little difference between adding sun- to an etched rune and just tracing the rune again and detonating the trace first. ...But for the reasons discussed above, I can't see why you'd be unable to. You can equip two fire resist rings even if it's not beneficial, right? And it frankly doesn't seem all that strong to be able to stack runes of the same type on a target, as long as you can only affect a given target once per invoke with a given type of rune. It could just be that sun- itself is ill-conceived and doesn't understand the rules; the damage scaling on runes certainly doesn't follow the damage scaling already in the game, so it wouldn't be the only thing in the class.
So which is it? Does the duplicate effects rule only prevent you from stacking the benefits of duplicate effects (while still letting you have multiple useless instances of the effects active), or does it also prevent you from having multiple instances of a given effect active?
There was a discussion on duplicate effects fairly recently. There may be some good information in there to check.
There was a discussion on duplicate effects fairly recently. There may be some good information in there to check.
This thread is worrying, because I actually see no consensus answer—especially not one that applies to cases like "I invest two Charms of Resistance (Fire)," which nothing seems to prevent. (To be clear, the issue isn't that I think they stack—that's clearly not what happens. The issue is that this seems legal, and it would create the exact kind of redundant duplicate effect that seems disallowed by the rules interpretation under which a second cast of mystic armor replaces the first.)
There's a lot of "of course you can't have stacked casts that do nothing, that'd be dumb," with no real arguments for it. But I don't see much direct discussion of the semantic issues in question, which are
1) What does "apply" mean in context, here?
2) Does "only benefit from one casting" mean only one "sticks" and the second overwrites the first, or does it just mean both stick and you only benefit from the better of the two?
The spells you mention at the bottom are things the caster can only have active once; it's a caster-based limit, not a recipient-based limit. It's not a limit on the amount of times it can be active on one person. It is open whether or not multiple casters could each put one rewrite possibility effect onto the same person.