Ok, a few things.
It is entirely possible to have multiple instances of a trait that doesn't stack.
Regenerate X
This object heals (removes) X damage each Upkeep Phase. The Regenerate trait does not stack or combine with other Regenerate traits. If an object acquires more than one Regenerate trait, use only the highest one. The Finite Life trait prevents regeneration.
The rules don't say that you can't gain multiple instances of a trait, just that if you do, only the highest one counts.
[Quote="Rules]
WHAT IS A TRAIT?
Spells, creatures, objects, and attacks may all have a number of special abilities called traits, printed on their card. These traits may allow you to act in ways not normally allowed, such as moving farther during an action phase.
Traits are listed on the lower half of a spell card or inside the attack bar. Some spells and abilities may also add traits to an object. For example, you might cast an enchantment to give a creature the Fast trait.
Many traits are followed by a value. For example, the Highland Unicorn has the Regenerate 2 trait. This number shows the strength of the trait. Regenerate 2 means that the Unicorn will heal 2 damage every Upkeep Phase.
Sometimes, this strength value has a plus or minus sign. Multiple traits of the same kind with plus or minus signs can be combined on the same object. So, if a creature with Lightning +1 is enchanted with a spell that gives Lightning -2, those two traits combine and the creature now has Lightning -1.
If a trait does not have a plus or minus sign, it does not combine with other traits that have the same name. Only the trait with the highest value counts. If the same unicorn was enchanted with a spell that gave it Regenerate 3, it would only heal 3 damage every round, not 5.
In these rules, and on the spell cards, traits are always capitalized. See the Codex at the end of this rulebook for a list of all trait effects.[/quote]
So, if there were a rule that prevented you from gaining multiple instances of the same trait, this is where that rule would be. But instead there is a rule that says that if you *do* have multiple instances of the same trait, only the highest one has an effect. And there's no rule here that says that e.g. two instances of Regenerate 2 is somewhow different than one instance of Regenerate 2 and one instance of Regerate 1, because traits without +s don't combine.
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Also, I think it's worth distinguishing between a trait and the *effects of* a Trait. You're right that often when the rules want to make it clear that the trait goes away, they say that the object "loses" the trait. And also that these traits which "cancel each other out" seem different from that.
But it says that Slow and Fast cancel each other out as traits, not just as effects. If a creature that has the Slow trait gains the Fast trait, those two traits cancel each other. Why decide that this means the traits stay but we ignore the effects? That seems like the sort of thing the rules would explicitly note, if it were intended.
But why would that be the intended behavior? The rules aren't a trick on players. Why have a technically Slow creature that can move and attack?
Slow
This creature is very slow. If this creature takes a move action, its Action Phase immediately ends. It does not get to take a quick action after moving. If a Slow creature gains the Fast trait, both traits are canceled out.
To me, that sounds a lot like Lightning +1 and Lightning -2 becoming the Lightning -1 trait. They have different names, so it's not the identical situation, of course. But doesn't it seem reasonable to describe a melee +1 trait and a melee -1 trait as "canceling out?" And if you instead want to propose that they combine into Melee +0, why are we calling it that and not Melee -0?
I mean, if a creature were enchanted with both Cheetah Speed and Enfeeble, and there were a spell that could only target creatures with the Slow trait, why would the system be set up intentionally so that the enchanted creature would be a legal target?
I guess it comes down to what we think "canceled out" means. I'm inclined to treat it is "is stopped and has no further effect" similar to other uses of the word cancel in the rules. I mean, it's pretty clearly natural language, not strictly defined jargon, so it's not 100% clear. But all jargon has to have, at root, some sort of natural language explaination. And given the lack of exact definition of this use of "cancel out" why go with the interpretation that causes weird rules issues?