What's the deal with redundant mods?

I have been wondering this for a long time, and I really want an authoritative answer:

An example: Skybringer

* Energy +10
* Lightning damage: 11-22 (Requires 9 Air Magic)
* Halves skill recharge of spells (Chance: 20%)
* Halves casting time of Air Magic spells (Chance: 20%)
* Halves casting time of Air Magic spells (Chance: 20%)
* Health +30

There are two HCT20% mods. I have seen this on numerous items. Are these stacking probabilities? If not, why is the same mod applied twice?

What's the definitive answer, if any? Has this been empirically tested?

The HCT20% mods do stack, but you won't get 40%, instead you'll get 36%.

Not redundant.

Excessive ^_^

Because you can get achieve 2x20% recharge and 2x20% cast time on wand/focus sets, anet thought they should add that functionality to staves...

This much I figured... that it is 20%, then 20% of the remaining probability, being 80%.

Thanks everyone for clarifying.

Just in case someone asked why 36% and not 40%:

Consider each 20% half cast mod as a test. With redundant mod, the test is done twice.

A spell won't be cast faster if it fails both tests.
The probability of failing both tests is 0.8x0.8=64% (0.8 chance of failing the first one, and among those 0.8, another 0.8 chance of failing the second test).

So you get 36% chance of getting at least one half casting mod to trigger.

Using the same logic, you can get both mods to trigger, with a 0.2x0.2 probability, which means 4% of Air spells you cast will be cast 4 times faster.

Dual 20% fast cast air is a beast, seeing as air builds are kinda spammy, run it and see the difference

Got a couple of free skybringers, if anyone wants, PM Skuld Tyyra

Agreed with Skuld, it is nice with air.

Also, its listed that way because its equivalent to a 20% inscription along with a 20% adapt staff head. The mods are listed twice because that's how a gold version of the staff would list them.

I tend to give something similar to my MM heros. Razah it helps bypass the lack of Bloodstained and MoW/Olias it just helps, specially with the obsessive Death Nova spam.

fast cast mesmer + skybringer...

your guess.

Erm... just to straighten some folks out:

The odds are NOT 36% for 1/2 cast time and 64% for normal cast time.
They ARE 4% for 1/4 cast time, 32% for 1/2 cast time, and 64% for normal cast time.

Both mods check independently. The second is checked regardless of whether first one succeeds or fails.

Half recharge works slightly differently. Although the checks are done in the same manner, a hard cap of 50% reduced recharge time is then imposed, so that 4% chance of 1/4 recharge is bumped up to 1/2 recharge time, and you effectively get 36% and 64%.

To recap:
2x Halves Cast Time 20% gets you: 4% 1/4, 32% 1/2, 64% normal
2x Halves Recharge Time 20% gets you: (effectively) 36% 1/2, 64% normal

thought they changed it so you can't get 1/4 cast times anymore.

I definitely wouldn't say redundant - ever since I got a Nightbringer, I've noticed a huge difference in my ability to raise minions, combined with Bloodstained.

I run the 20/20/20 on all my Ele staves now. Being a nuker is so much fun when you can zap off Meteor Showers in double quick time every now and then

They did Chthon is incorrect.

They should stock furious mod like this, that would give us mending wammos 20% furious instead of 10%!

Ahh, but Warriors have skills that increase their adrenaline gain, whereas other classes (with the exception of Mesmers) don't have skills to increase their casting rate - increased rate is soley gained from weapon mods and insignias (Bloodstained).

Therefore, 20% Furious would be WAY overpowered - and this is coming from a player who luuuurves her Warrior toon!

Nope. You can still hit Quarter casting time.

Yeah as far as i know they only changed it so you can't get 1/4 recharge, but 1/4 cast is still possible.

But then again i haven't played my ele much recently, or i'd probably have noticed a 1.25s cast Meteor Shower.

I have a 20/20 HCT and HSR on my fire wand, and my offhand. Every once in a while I can get off a meteor shower almost instantly...it's definitely still possible that they are both put into effect.

I did that once. I was like 'WOW' how did I do that? Then I found out.