One solution would be to turn off "Auto Import" in Lightroom Mobile on either your iPad or iPhone. The photos are being duplicated across the devices using Apple Photos, so you don't need both devices to import them into Lightroom.
I also have noticed this problem and had concluded that I did not need to import on multiple iOS devices unless there is a starter Adobe solution.
Thank you for any help
I have a similar problem. I have Lightroom mobile on both an iPhone and iPad. On both devices i have have "Auto Add" disabled, but i still get two copies of every iPhone picture added to my Lightroom CC. Any ideas on how to stop this? In addition to the duplicates being annoying, there are often pictures i take with my phone that i do not need to save.
I do not know how or why LRM is auto importing if you have it turned off.
I use a plugin called Teekesselchen to sort out the duplicates.