Same issue here. A duplicate layer was added to the legend (after I made some edits to the layer) but there is only one of that layer in the map. See the parking layers below, the top "Parking Garage" and "Parking Lot" are the original layers, and the duplicate layers are at bottom, which also show the heading.
![Screen Shot 07-12-24 at 05.40 AM.PNG Screen Shot 07-12-24 at 05.40 AM.PNG](https://community.esri.com/t5/image/serverpage/image-id/109649i4EA5FB5716EF932C/image-size/large?v=v2&px=999)
When I attempt to toggle off the duplicate layer in the legend, it stays checked in the legend items and also remains in the legend, but the original layer is turned off and removed from the legend.
![Screen Shot 07-12-24 at 05.42 AM.PNG Screen Shot 07-12-24 at 05.42 AM.PNG](https://community.esri.com/t5/image/serverpage/image-id/109650i6779C8B23D7E1A94/image-size/large?v=v2&px=999)
Removing the duplicate legend item also removes the original item.
The only solution I've found is to remove the item from the legend and drag it back into it from the TOC.