Perhaps and Esri dev can elaborate on this, but I believe it has something to do with how the JS 4.x API is rendering feature layers.
You can see sometimes as you pan/zoom that feature layers are loaded and rendered as "tiles", and when a feature spans a "cut line" between tiles, it tends to repeat its label.
I believe it is a known issue, but I'm having trouble finding the precise bug report.
- Josh Carlson
Kendall County GIS