Still a problem. Legend has duplicate rows for grouped values using Map Viewer (Beta), but it's correct displayed in Map Viewer.
Using the Map Viewer (Beta) map in a webapp built on ArcGIS JS API (4.22).
Stuck with ArcGIS Enterprise 10.8.1 at the moment, later releases fixes this perhaps, but the bug was reported in a decade ago, not too hopeful.
Still a problem in 2023. Issue is visible within Experience Builder using the Legend Widget. Similar to ParAman the Map Viewer legend shows up fine, but the legend in EP shows duplicates where symbology was grouped.