if i conquer zaun warrens with a unit with last rites can i discard and draw a unit and play the same unit with last rites

No. A unit you discard from the Zaun Warrens trigger can't be the one Last Rites plays — Last Rites' target is locked in when its trigger goes on the chain, before Zaun Warrens' discard can resolve.

When you conquer, both triggers go on the chain simultaneously and you pick the order (Rule 383.3.d). But the target for Last Rites ("a unit in your trash") is chosen during finalization of that trigger as it's placed on the chain (Rule 402.2), not at resolution. Whichever order you stack them, the discarded card isn't in your trash until after the Last Rites trigger has already been placed with its target chosen. So Last Rites can only play a unit that was already in your trash when the conquer happened.