Current.

I had half an idea there were some contrived circumstances involving Dispel Magic that made that reading as crazy as the other one, but after further consideration I can't think how the version that means keeping track of the "original strength of each spell" could ever be reasonable.

Since your first strength casting of False Life was weakened, you've replaced it with the new casting.

I actually go with a different interpretation, though I totally admit it is a judgement call on my part as I can't tell for sure from the wording of the section of the rules. I check the strength at casting and that sets the order, changes to the value once cast don't change which one is active.

The reason for this is Mirror Image. Using your interpretation (which one is active changes based on changes in value during spell duration, which could also be a valid reading of the rule), one can cast a lot of Mirror Images on oneself, and end up with essentially an unlimited supply of multiple images, which makes the spell extremely powerful at high levels (basically you have to give all opponents Trueseeing, or they just have miniscule to hit chances since they will be attacking mirror images almost all the time).

Example: Bob has Mirror Image cast on himself 5 times, with 8 images for each casting.

My interpretation: The last casting is active. As the enemy knocks of mirror images it stays active so Bob goes down in images as they randomly get whacked, and the enemy starts hitting him more frequently after just a couple swipes. At high levels a tough enemy can probably get a couple hits in over the course of a round or two. At some point all the number of images gets low enough that Bob dismisses the spell (it has a (D), at which point the next Mirror Image down the chain becomes active.

Your interpretation: The largest number of images casting is active. An enemy knocks one image off of Bob and the next spell down the chain becomes active so a replacement image immediately appears. It takes a LOT more attacks to clear out the images (5 succesful attacks on images gets you to the point where Bob has 7 around him instead of 8..., vs 3 around him under my interpretation).

Not saying you are necessarily wrong, just that's the logic I used for why I went with 'at casting' for determining which one was active rather than 'constant checking'.

:)

I think the previous castings have to be treated as truly "irrelevant" (which is used but (I don't think) defined anywhere). They are not merely inactive until they become the strongest effect: they do nothing but show up as Detect and (funnily) bogus Dispel targets.

Like in my False Life example, taking 2 more damage will not give me one extra hit point as the first casting cuts back in.

Like in my False Life example, taking 2 more damage will not give me one extra hit point as the first casting cuts back in.

yes, but that wouldn't happen under my interpretation either. You'd have to get the spell off, which does seem like it would happen when all the points were used up given the duration wording of the spell, which would definitely be a problem since then you could stack a gazillion of those and have ridiculous hit points. So my interpretation while not having the specific flaw you mention would be screwed with stacked False Lifes regardless. Oy!

What is possible is that they really only meant the Same Effect More than Once in Different Strengths entry to apply only to spells that gave bonuses/penalties. Unfortunately Same Effect with Differing Results gives the problem outlined above, where a stack of those spells would come off one at a time, essentially allowing ridiculous HP. That could just be a flaw of the spell however since it is the specific duration for it that is problem. I don't think One Effect Makes Another Irrelevant applies to either of our cases, but the text doesn't do a good job of giving an example (sort of like the Sunburst spell's example of vampires as what sort of undead are destroyed by it doesn't really clarify what counts as harmed by sunlight).

:) Ultimately what it is is that the strict rules don't have the level of nuance to get the behaviour the designers want (excess HP don't stack, you can't cue them up, but after you've lost them all it can be cast on you again before the duration would have been up is obviously what they want to have the spell work like, but that doesn't quite fit any of the categories...).

I'd just rule it as the new casting cancels out the old casting. To me that's the correct ruling.