It's because people have randomly put in letter/number combos and gotten a code right. Not to mention there's a generator for them. I had the same problem a couple times and looked it up on Google, that's the info I got. Nothing you can really do about it though :\
I suggest getting a 4 pack with the Destiny box because when redeemed it gives you 4-6 focused light (the "-6" being that I didn't know if I had any waiting at the PM cause 1 for sure hadn't shown up yet.)
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I know about that. It's happened before and when I posted in this forum about it, bungie took care of it.
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Noted.