We have found that when ideas get posted through out control portal, they may result in multiple features.
There is currently no mechanism to take 1 idea and either promote it into multiple features (not sure what that would look like), but at least allow it to be linked to multiple features.
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In a scenario where a customer enters, we would suggest promoting the idea to a feature to provide the first link. You can then create a new feature - and within the actions menu of the new feature, you can create a "related idea." This would be the best way to track multiple ideas across multiple features.
We are unlikely to make updates in this area because of the way that status gets updated back to Ideas. Currently, idea statuses are mapped to feature statuses in a way that when a feature is shipped, this is also reflected in the idea status. By linking an idea to multiple features, this would cause discrepancies in how the idea status would get updated.