Hello,
I am actually experiencing this issue as well. I have 4 related items (web map, web app, layer, service definition). The web map is in the root folder and I'm trying to move it into the sub-folder where the other 3 items are and I get this message:
I've looked at the service URLs for all 4 items, as well as the IDs and none of them match, so not sure what the duplicate 'thing' is that's preventing me from moving my 4th item.
I will try Erik's suggestion of using AGOL Assist, but want to see if there's a resolution for this issue too.
Thanks,
Dan