Handling of duplicate ideas has been a very tricky thing. The way we solve this is by promoting multiple ideas to the same feature, making the feature a kind of 'control point' where all of the ideas come together.
The problem is that this introduces a lot of manual labour to keep fields in the feature up to date against the fields in the idea - namely tags and custom fields that we use (customer asking is the biggest one)
Ideal flow:
Select a set of fields in the idea that map to fields in the feature
whenever a new idea is promoted to an existing feature, this subset of fields would be added to the fields in the feature
Another option:
Create an automation that is triggered by 'when an idea is promoted to a feature'
Add values from field X in the idea to field Y in the feature
This would reduce a HUGE amount of manual work that currently goes into keeping these fields up to date