I can't remember the last time I ever had to worry about duplicates but recently I imported pictures manually from my camera roll and it clearly has imported hundreds of duplicates (with identical filenames mind you). This exclusively happened to HEIC and JPGs from my iPhone's camera roll so it feels like a bug specific to iOS perhaps.
All this brings up the very odd choice of not offering the same deduplicate feature available within the import flow *after* the images are already in the library. In my case, that's hundreds of photos I have to hunt for manually, when the process would be instantaneous if the dupe finder was available outside of the import flow.
It would also be incredibly useful to be able to group photos from the library that are not *quite* duplicates but very identical (think bursts or multiple edits of the same shot) and allow me to find them, pick the ones I want to keep and either delete or mark the others as rejected.
I'm not quite sure what version of Lightroom for iOS I was running when the duplicated import issue happened but I'm currently on v7.1.0 if that's any help.