Without remembering everything that's in the backlog, it's hard to know if there's already a feature/epic/initiative that is the same or similar to an Idea
Idea review takes longer and requires deep knowledge of the existing backlog.
The most common disposition of ideas is either - 'it's already planned', or 'we're not getting to it anytime soon'. We just want to let users know which it is as quickly as possible.
When an Aha user is reviewing an idea, suggest related features/epics/initiatives - not just related ideas.