Reference guide definition of unless
QuoteIf an effect says, “unless,” is it giving you an option to resolve the effect following “unless.” However, if you choose not to resolve that effect, you must resolve the effect preceding “unless” instead.
Investigator that has an Amnesia condition, rolls for Plumb the Void , passes with 1 success and resolves effect on the back:
QuoteDiscard this card unless you gain an Amnesia Condition.
Now you can't have two Amnesia Conditions.
QuoteAn investigator cannot have multiple copies of the same Condition. If he would gain a Condition that he already has a copy of, he does not gain another copy of that Condition
There are now two schools of thought:
A. You have to discard Plumb the Void because you did not gain an Amnesia.
B. You can still pick and resolve "gain an Amnesia Condition" since the choice does not rely on actually getting it (like it could be blocked with Wendys ability). Even if you don't endup with a new Amnesia next to your Investigator, the "gain" was still resolved fully, thus Plumb if not discarded.
Which is it? I'm in the B camp by the way...