So here is mmy understanding based on the rules.

If I have an Asha Greyjoy in play and an Asha Greyjoy in my dead pile, Aeron Damphair can return asha to play as a duplicate.

Is this correct?

Yes, provided you don't have another Asha Greyjoy in your dead pile (from another Heads on Spikes, since I assume the one you're trying to use Aeron on was hit by Heads on Spikes).