You will first need to check if a duplicate is attacked before applying other effects.
Mirror Images states:
...roll a d20 to determine whether the attack instead targets one of your duplicates...A duplicate can be destroyed only by an attack that hits it. It ignores all other damage and effects.
Among the Dead specifically states (emphasis mine):
If an undead targets you directly with an attack...
Since Mirror image actively changes the target of the attack, its outcome takes priority over pretty much any other effect on your person. The duplicates ignore all other damage and effects aside from attacks, meaning that they do not share your buffs. And since Mirror Image actively changes targets, effects that resolve when you are targeted for an attack rely on whether or not a duplicate is hit first.
In a more general sense, if two effects on your person appear to be triggered at once, look for wording like this in the effect description to suggest which effect has the more specific rule. Then follow the mantra specific beats general. If the wording is still ambiguous, then the decision falls to the DM (PHB pg. 5):
The DM determines the results of the adventurers' actions and narrates what they experience.