The Navigation-Popups-Gadget also supports the preview of references/footnotes. When it is used together with the Reference Previews feature from the Page-Previews (Popups) extension, it shows two popups when hovering a reference.
This should not happen. I guess we should disable Reference Previews automatically when we detect, that the user uses Navigation-Popups-Gadget . Similar to what we do when we detect that the user uses the reference tooltips gadget.
Note, that there's already a generic hint on the preferences page where you can toggle the Reference Previews feature, that informs about the possibility that gadgets might conflict.
Note, that in T267846: Investigation: Using ReferencePreviews together with NavigationPopups we did not consider this, because there the whole extension was disabled when the Navigation-Popups-Gadget was enabled. Now that's different.