There is a post about this item: viewtopic.php?t=31659
But this doesn't solve the issue.
Just created a new scene/group. And it get an already used idx. Created in Incognito mode of Chrome.

I'm already 4 year using Domoticz. Last week I updated to the latest stable version. I've seen already there where some idx double in the devices. Using Dashticz gives problems if I want to use a idx from a scene/group with the same idx as my P1 meter. See attachment for a detailed screenshot Is ...