This is expected behavior. If you have an instance of the AnimBP in the game AND you have the AnimBP open for editing, then you will see two results output: A success and a Failure. This is because the preview window in Persona is a running instance of your AnimBP.
You’re expected to use an IsValid? check in AnimBP Event Graphs. See the Third Person Template project for an example of this.
The isValid check essentially lets no signal pass unless you have a legitimate Parent Pawn. That stops the AnimBP preview instance from triggering anything. Does that help?