fully upgrade 1 exotic weapon
i have 2 fully done
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I may be wrong here, but since that was one of the original achievements, it may require it to be done with a year-one weapon. If you have any, you could try one of those, if not, you could buy Xur's Legacy engram tomorrow, and upgrade whatever he gives you.
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Try going to settings inside destiny and sign out then sign back in, once choosing a character it should trigger.