It's really a pain when I'm using QS to get rid of all these items in my Inventory and I often pick up plenty of wearable items I like or might want to add, and it's so hard to keep track of what you already have that I often keep getting the message "X" was not added your wardrobe as it is already there!" I'd like there to be a way to better keep track of what I already have so I don't keep getting that purple monster I hate so much snarling in my face, and I wondered the same question when I was fresh and new to Subeta...Why do we have to have one and only one of a certain wearable in our Wardrobe? It can really be a nuisance.
I think it would be because there really isn't any point in owning more than one of an item in a wardrobe. Not to mention that wardrobe achievements would be too easy as all people would have to do would be to buy up a bunch of some ridiculously cheap wearables such as angel wings for example and then place it in their wardrobes for easy achievements.
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Buying discounted Kumos items and intelligence boosters.
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I think the achievements are the main reason. As well as the fact that the Wardrobe probably operates like a Collection and has "slots" for each item to be filled, I noticed this by looking at the Achievement page in the Wardrobe section, note the ones that require specific items. Once the slot for said item is filled, another like it cannot be added because the Wardrobe recognizes as it being the same item. Much like a Collection, except with the Wardrobe you can remove items. Still, a way of keeping track of what items you already have WOULD be nice.
A long time ago multiples were allowed in the wardrobe, and it was a royal pain to clean out or go through the wardrobes with all of those multiples... especially since there is no reason to have multiples floating around in there. It was halted before the achievements came out, if I remember correctly, although that does further reinforce the need for the rule. Personally, I would much rather see that monster than go back to that hellish existence. Maybe try using Quickstock? That way you'll only see the monster once for each load of items...
The account search already lets you know if it's in the wardrobe. If you're unsure if an item is in your wardrobe just use that, that's what I do.

Like said you can use the account search feature OR you can go into your wardrobe and search for the item there. I just use QS to mass add things to my wardrobe. I choose 'Add to Wardrobe [all]' and whatever I already have just stays in my inventory so I can just go right back to QS and add them to my shop. That's how I usually figure out if I already have a certain wearable or not. So if we could have multiples it would make it a lot harder for me to figure out what I already have.
I get so annoyed with wasting sP on something I already have. And it's such a pain to Account Search everything when you just want to shop! I wish there was an outline like the one for what's on your wishlist but in a different colour, so you know you have the item in Wardrobe/Collection. It could be the same colour for the wardrobe and all the collections. But that said, I'm glad it won't let me add two of the same item because that would mean I'd have stuff I don't need in my wardrobe and that drives me mad lol.
I pretty much agree with everything you said. If there was an outline, like purple or orange, we'd know it's something we already have and we won't have to waste sP on it.
I would absolutely crap a brick for the lag that we would face for an account search to be run for EVERY item on EVERY page, just for an outline to tell me something that I should be perfectly able to find out on my own. It may be ANNOYING to run an account search as it is, just as it may be annoying to have to resell something that you didn't realize you already had, but slowing page loads that much would be an annoyance for everyone, instead of just you. If you're RSing an item and don't have time to do an account search because it sells fast, then chances are fair it also resells for a higher price, so you're losing nothing to buy it anyway. If you're buying an item from user shops or one with a lower rarity that won't sell so fast from site shops, then you have time for an account search.