When I click the "open in mapviewer" for this geojson item, I'm presented with a map having duplicate layers.
I was able to open the layer up with just one layer. Maybe this was added to an existing map. Try clicking open in new map when you open the feature layer to ensure that you are starting on a fresh new canvas.
Ok, that produces what I expect, no duplication.
Occasionally I'm able to get it to misbehave if I double click real fast on the Open In Map Viewer
Retrieving data ...