Did you manage to resolve this or find a workaround? I noticed this in my app as well.
From what I can tell, it happens only when using TileLayers and usually related to adjusting opacity of a layer above the bottom layer.
Here's a pen I made to try to duplicate the issue (you need to sign in to arcgis online to load the Ecophysiography layer): https://codepen.io/broms/pen/BaKdXaj
When I load the pen, the symbology is correct, but as soon as I pan the map, it duplicates. Removing and re-adding the layers fixes it, but then adjusting their transparency causes problems again (either the layer disappears entirely from the legend, or the symbology duplicates)
I believe it's a bug in either TileLayer or in Legend. Happy to try to gather more info for a bug report/fix.