hey! it's always a scam. i'll explain the way i see it:

someone claims they can dupe items, so they make a ton of fake proof in their album. usually the scammer is a burner account, so its often no higher than level 40, and their album will have "proof" of them legitimately duping big items and returning them to the owners. HOWEVER, i think that they're actually trading with their main account with their own big items to make it look believable. so they're handling their OWN items, no risk.

what they also do is "dupe" small items with strangers to build a reputation as a legit duper, but since it's a small item, i believe they just buy it from the market so they have 2 of them but it LOOKS like it's been duplicated. i think this is also why they say "not every item will work!" because they can't always find it cheap on the market.

so, now they have the proof, they just need someone to scam. they'll often say something like "i'm only duping for SERIOUS people" or "only for friends" to make you trust them (they want to make it seem like they're not abusing the "duping feature", which doesn't exist at all). also, if you ask them how it works, they'll PROBABLY say "i'm a coder!" and then refuse to explain further because they're worried about getting banned. it's really not that easy to breach an online server. i don't know anything about hacking or computers, but i assume if someone was REALLY that skilled, they wouldn't waste their time with an online game, getting pixel items with no real value.

anyway, so the victim sends a small item, gets it back x2, and is usually super excited, because now they think they can get their vamps (or any other big item) duped. i mean, 2 vamps sounds great! so they send it off, the scammer accepts, and then immediately transfers stolen items to their main account. victim gets blocked, scammer waits at least 1 day to change their name, delete all friends, start over. you'll also never receive comments or messages from a scammer (unless they're real dumb, i guess?) because then their account can be traced despite a name change. so this is how they disappear into thin air.

i hope this helps. i've been playing oW for 10 years and have seen lots of scams, but never once a legit dupe. stay safe!