Sorry for being ambiguous. I defined my semantic model via text (same as quite a bit of my items as a relict from my OH2 days) pretty much as you described in the initial post.
However I did update the semantic model groups based on your description with the uiSemantics tag and I also had to update some of the item definitions as well (mostly lights with both dimmer and white color temp). For the latter ones I updated the item definitions quite a few times and it seems every time I did it (via “Add items from textual definition”), another instance of the item is added to the widget.
As I wrote the weird thing is that I see this also in the developer view of the widget.