Activate Adarkar Valkyrie by tapping it, targeting any legendary creature you control.
Cast Dack's Duplicate by paying ({2} magic symbol) ({U} magic symbol) ({R} magic symbol) , choosing to have it enter the battlefield as a copy of the legendary creature targeted in step 1.
Put the legendary creature into your graveyard due to the legend rule.
Adarkar Valkyrie and Intruder Alarm trigger.
Resolve the Intruder Alarm trigger, untapping all creatures.
Resolve the Adarkar Valkyrie trigger from step 4, returning the legendary creature to the battlefield from your graveyard.
Put Dack's Duplicate into your graveyard due to the legend rule.
Adarkar Valkyrie and Intruder Alarm trigger.
Resolve the Intruder Alarm trigger, untapping all creatures.
Holding priority, activate Adarkar Valkyrie by tapping it, targeting the legendary creature.
Resolve the Adarkar Valkyrie trigger from step 9, returning Dack's Duplicate to the battlefield from your graveyard as a copy of the legendary creature.