ArcGIS Online Labels are Duplicating

[GISAdmin7]

I have a webmap with street labels enabled. Last week I noticed that the labels had started duplicating. These labels are sourced from a map service on our ArcGIS Server version 10.9.1. It's been months since we last edited the service or it's .mxd. The labels appear correctly in the .mxd and when I select "View in ArcGIS Online Map Viewer" from the REST Services Directory. The labels start repeating when added to a webmap. 

Here's what I've tried so far:

1. Overwriting the service
2. Turning off Maplex
3. Creating a new .mxd and service with only the street labels
4. Turning off the halos

None of that has worked. As soon as the labels are added to the webmap they duplicate and ignore the labeling rules. I've attached pictures of what the labels look like in the .mxd and what they look like in the webmap.

[ChristopherCounsell]

Your best solution may be exploring publishing the same service from ArcGIS Pro.

10.9.x is the last version of ArcGIS Enterprise that supports publishing services from ArcMap.

With ArcMap heading into maturing officially last month, it wouldn't surprise me to see increased incompatibility between produces/services and ArcMap or ArcMap created content. Particularly for Map Viewer. You could test to see if the labelling works in Map Viewer Classic.

Either way, I'd use it as a good reason to push the move to Pro. 

[jcarlson]

On AGOL, are you viewing the Feature Service or the Map Service? The two will have vastly different label capabilities.