I spent 155 Strange Coins today on legacy engrams trying to get the last remaining year 1 secondary (4th Horseman). I already have the year 2 version and it is my belief that is the reason I could not receive it today. The cause may lie in the way that the RNG works. I believe that the smart engram is looking at the Grimoire card on my account. Since I have the year 2 weapon, the Grimoire card is already unlocked.
Please look into this.
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How, It has been stated by Bungie that the legacy engrams are highly weighted to give you an item you do not already have in your collection but it is not guaranteed. [quote]These Engrams contain Year One Exotics (at Year One Attack and Defense levels) for players who want to finish up a Collection. [b]They strongly bias towards Exotics not already in your Blueprint Collection[/b], to help fill in the blank slots [/quote] https://www.bungie.net/7_Bungie-Weekly-Update---09172015/en/News/News?aid=13575