A lot of you probably already know, but PSA for those who don't. Apparently you can only equip one at a time...
If you're already wearing/using an exotic, [i] and don't have something to swap it with [/i] , then don't bother purchasing exotics from Xur.
Just like I think there should be an 'are you sure' with every purchase, so that you don't buy things by mistake (I've bought shaders I didn't want by accidentally bumping the button), there should be a 'you already have an exotic equipped, are you sure you want to buy another?'
I searched it and apparently people have been upset about this for a while. So yeah... Your attention please, Bungie...
Edit 1: I've been around since year one. I don't grind like crazy. I work 6 days a week and am not a heavy gamer. My point is, there should be a message where it informs you, and you could choose 'yes, don't show again.'
Edit 2: Thank you to everyone who's kindly helping bring me up to speed, rather than bashing me. I understand I should have known this by now, but various circumstances kept from playing much since March. So thank you for being kind.
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As far as Exotics go ... you can gave several for each slot (couple of helmets, couple of chest pieces ... couple of each weapon) you just switch them out as you fight different enemies depending on what the perks are for each Exotic you have ... you can own as many and use as many as you wish .. only one at a time for Armor and Weapons.