It's possible to get duplicates if you already own all of the legendaries in a set
This, but OP said it like he got Aluneth as a card reward. I think the "no duplicate" rule goes for pack openings only, so if he's getting a legendary card from Arena, he might get a dupe.
Legend with : S65 Freeze Mage, S57 Maly Gonk Druid, S57 "Okay" Shaman, S53 Boom-zooka Hunter, S53 Maly Tog Druid, S52 Wild Tog Druid ft.Blingtron, S50 Quest Rogue, S49 Dead Man's Warrior, S41 Wild Clown Fiesta Druid, S41 Hadronox Jade Druid, S40 Wild OTK Dragon Druid, S35 SMOrc Shaman, S33 Jade Druid, S22 Control Priest, S19 Control Priest
It's possible to get duplicates if you already own all of the legendaries in a set
Legend with : S65 Freeze Mage, S57 Maly Gonk Druid, S57 "Okay" Shaman, S53 Boom-zooka Hunter, S53 Maly Tog Druid, S52 Wild Tog Druid ft.Blingtron, S50 Quest Rogue, S49 Dead Man's Warrior, S41 Wild Clown Fiesta Druid, S41 Hadronox Jade Druid, S40 Wild OTK Dragon Druid, S35 SMOrc Shaman, S33 Jade Druid, S22 Control Priest, S19 Control Priest