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The odds of each glaive dropping is 0.05 so the odds of both of you getting the one you want is 0.0025 or 0.25% or one in 400.
I imagine at least 400 people kill illidan each week though so it's going to happen to somebody every week. (same numbers apply to getting both glaives in 1 run)
I don't think this matters nearly as much as you think it does.
"What are the odds that either of 2 people included in set [people trying to get it this week] got it" - ~5%
"What is the probability that a specific set of two people within the set [people trying to get it this week] will both be included in the set [people who got it this week]?" - math I don't feel like doing, but probably not 5%.
To put it into context, any card in a standard deck of playing cards has a 1/52 chance of being drawn, but the odds of a specific 2 (or five) being drawn during a particular game vary. This is why 3 of a kind beats 2 pair, because it's less common. If the odds of 2 people getting it on the same day remains the same (like the odds of 2 aces being drawn in a row still being 1/52) then math has failed to work.
This is a common misconception that stems from a true statement, "No matter how many times you flip a coin, it still has a 50% chance to be either heads or tails." That's true, but it's also true that the probability that you'll get, specifically, the sequence "Heads/tails/tails/tails/heads/tails/heads/tails" has a discrete value.
I'm just going to stop there because most people don't know what a probability distribution is and will just flame me about being wrong anyway.
But, realistically, it is pretty unlikely that you and your friend both got it in the same week. Congrats on a cool coincidence!
Statistics is what you're not good at. Math has nothing to do with it. Though I guess statistics relies on math...
Anyway, your chance to get an item doesn't impact the other person's chance to get it, nor does it impact your chance to get the second glaive.
People have this notion that percentages are additive or something, like if you kill a boss X times you're "due" to get a drop. Now, I understand in LFR there is a function to increase probability of drops if you haven't gotten anything, but nowhere else does that work AFAIK.
A good example was back in MC days I recall getting felheart robes (either one or TWO from the same boss) several weeks in a row, probably 10 weeks straight. Never saw nightslayer chest (the one I wanted). I raided MC for a good 7 months and never saw it. But that's RNG for you. Getting the same item 10 weeks in a row doesn't change the odds of seeing it the next week, just like flipping a coin and it landing on heads doesn't tell you tails is "due" or more likely the next flip.
Just as often as people never see the drops they want, other people are swimming in drops they don't need because they see them all the time.
Edit: I should mention that as the poster 2 up says, there is an independent probability if you were to zoom out and say OK what's the probability of both of these people getting the glaive the same day. It's just that you getting a glaive doesn't CHANGE the other person's probability of getting it. It's still 5% for each of you, it's just that if you combine that percents it's 5% of 5% for BOTH to get it. Each individual's chance is still the same.
Went and did Sunwell on my monk for the first time. Thori'dal drops. I proceed to open trade with all my hunter buddies and place the item in the non-trade slot. Sad, I can't even equip it. Lol, they hated me for a few weeks.
Sinthetik, Blood DK, GM, Exiled From Hell, US Premiere Daytime Progression Guild. 3/7 Mythic.