Somewhat tangential, but does anyone know what happens when two different instance of the same spell are trying to apply different effects?
If two different casters maintain Enhance Ability on the same target, one for Strength and one for Dexterity, what happens? Assuming they were cast using the same level of spell slot, by casters of equal level and casting ability, then I don't see anything to distinguish one as being more potent than the other. They're boosting different scores, so the choice of which one applies would matter.
If I try to apply a rule based on casting order, I get shenanigans. Saying the first instance takes priority means a target can prevent their self from being affected by a Suggestion or a Geas or Phantasmal Force by having an ally cast a version with a harmless effect. Or cast Bestow Curse on an ally, to give them disadvantage on attacks against the caster, and prevent anyone else from cursing them in the process. Saying the most recently cast instance takes priority fails for basically the same reason: now you can block the effect of another caster's spell by having a harmless version cast. Or you could block an enemy's Enhance Ability buff by boosting a score they don't care about--the spell doesn't say the target must be willing, and it offers no save, so why not cast it balefully?
I'm lead to believe that when the bless example was used, it was because the authors were thinking about spells that operate like bless when they wrote those rules. That is to say, spells of a fixed duration (+- concentration) and where each instance can have a different size but an equivalent kind of effect. In which case, the distinction between which of two equally potent effects wouldn't matter.. you don't have to know which instance of the spell is applying its effect, since you know they're all doing the same thing.
A very good set of questions.
I think the simple ruling on this is:
All versions of all spells affect at all times. Only the exact same effect (or boosted same effect) does not repeat. In that case, only the most potent version works (if same potency, do not take best of 2 rolls). To be the same effect, it has to be the same spell (special abilities by monsters are not spells unless otherwise stated, so multiple similar or same effects are possible).
So, someone could have Geas on them from an ally, but a different Geas from an enemy would also still work. DItto Bestow Curse since disadvantage against two targets would result in two different effects. Enhance Ability for both Str and Dex results in both being boosted.
Two different Suggestions could result in a conflict, so some simple alternating save until one spell is gone might be needed. A spell that results in 100% compulsion like Suggestion needs a way for only one of them to work at a time if and only if they conflict (a Suggestion in a higher level slot would be more potent and hence win in a conflict scenario). The other Suggestion spells duration still exists and if the winning Suggestion spell gets disrupted, the other Suggestion spell "turns back on".
A Suggestion that conflicts with a Geas, too bad. He will do the Suggestion and he will take damage from the Geas that day. Assuming that the conflict does not deal with harming the target of the Geas caster. Geas is still a Charm spell.
This way, every spell that is cast works, just like if there were only one spell there. The only difference is that two exact same effects spell basically have one of them turned off. Same spell different effects results in both spells working.