AFAIK every instance of a mirror is treated as a unique item, throughout Legend (not just agenda).
For example, any of the group-by panes will show every instance under each header.
I expect some workflows would need the current behavior but a pane option for “show only one instance of mirrors” would be valuable, especially for things like a flat-list Pane used to focus on a simplified set of actionable items.
Nearly all my workflows would be better if all instances were collapsed up into the topmost one in the document.