Only original skins are considered canon?
Some outfits from the rift=canon
Hallowed/withered blight skins=canon only on october
The store cosmetics=They are only to make your character look good.
Depends what you mean by canon. Lore-wise, I believe all the outfits are supposed to be lifted directly from the character's own memories (excepting recolours, perhaps) but I don't really see how that could realistically be the case with certain outfits. It's possible the Entity, or whoever creates the cosmetics in-universe, took more creative liberties with some outfits than others, but I think they're all meant to have some basis in the character's experiences in their previous life.
And the fact that each player has the same character can look different-game mechanic?
Basically, yeah. Lore-wise, there is only one of each character in the Entity's realm, but for the sake of player experience the devs decided not to place that restriction on the matches themselves.
The way I see it, all of the cosmetics are either relics from their past (all the cosmetics with a story behind them like Oni's wolf outfit and Claudette's lab intern outfit), mish-mash conjurings of various thoughts and memories (L4D crossover outfits), other survivor's stuff, or natural progression of the survivor/killer in the Entity's realm (Hallowed Blight, prestige). The devs said most cosmetics are found in the Fog, like how things are found in the Bloodweb.
Or they're just outfits and look cool
And the fact that we can meet several identical characters is a game mechanic?
And the clothes that this or that character is currently wearing, each player decides for himself?
Mechanically speaking, yes. In terms of the in-universe lore, though, I'm not actually sure who decides what the characters wear and when.
Maybe the player himself chooses what to wear this or that character and by lore?
The players aren't really part of the lore themselves, unless you're suggesting that the characters themselves choose their outfits. Which is possible, but it's also possible that the Entity dresses them. We don't really have enough information about how it works to draw any definite conclusions around that.
Not counting the Rift which specific survivor theme and Cosmetics are 100% from their backstory so are canon.
I'm not talking about general cosmetics that pop up in the Rift and talking about outfits that are about the tomed centred characters
Default and alternative cosmetics are also considered canon. For example both Min's Laser bear outfit and her day off outfit are both canon.
Event cosmetics for the most part do seem to be cannon things like the lunar New Year the Halloween event is a little bit more debatable due to how not knowing 100-percent how the blight works.
We do know that not counting gameplay reasonings (player choice) in the lore there is only one of each survivor and killer with the exception of the Legion because they are literally four individual people going under gang title ( but even in that case there is only one Joey, Susie, Julie and Frank)
This makes the blight cosmetics very hard to pin down as the outright mutate The Killers but no ways the killers still retain their basics looks. So it's possible that for blight mutations disappeared when the flowers do and come back when the flowers return.
This would explain how both a mutated version can exist whilst The Killers can still retain their base looks.
Store cosmetics are a little harder to pin down.
There seems to be free different types of cosmetics.
Memory-based- visa once directly pulled from the survivors memories and are most likely canon. Feng's tournament outfit and Meg's tennis outfit both definitely seems to be pulled from their past.
Creative liberties- outfits that seem to be relatively pulled from the past but also seem to have some creative liberties by the hand of The Entity or whoever makes the cosmetics. This could be the explanation why there are alternate colour/style variance for the same outfits. All of Feng's geek chic outfits. It's possible one variation could be canon while the others are creative liberties or they can all be canon.
Pure aesthetic- these are outfits that are clearly not canonical but they just look nice or intimidating-for example Sea Monster Trapper or The Candle Wraith
But in the world of Entity, is each character individual?
That is, there are no several identical characters, how can one be found in the lobby?
Maybe the Entity got bored of the pain and suffering and decided to play dress up with the survivors and killers. Honestly, it's getting a lot better at it. All the upcoming cosmetics on r/leaksbydaylight look awesome compared to things like Claudette's circus outfit or Nea's blue bob disaster.