I just recently upgraded to ArcGIS 10.5.1 and I think I've found a bug.
It's happening on 2 machines here running 10.5.1 We haven't upgraded any others.
In the Placement Properties - Label Density tab of the Label Manager when using Maplex Label Engine if you check Remove Duplicates then click options and enter any number, press OK and get out of the manager then go back in, the number is now 0. Try to change it to another number and it reverts to 0 when you go back in.
Is anyone else experiencing this?
Southern Georgia Regional Commission
This issue has been fixed and will be included in the 10.6.1 release of ArcMap.
In the meanwhile there is a workaround using styles:
From within ArcMap open the 'Customize menu' -- 'Style Manager'
Select your user style and double click 'Maplex Labels'
On the right hand pane right click -- New -- Label...
In the dialog that appears click 'Placement Properties'
Select Polygon from the 'Feature Type' drop-down at the bottom.
Navigate to the 'Label Density' tab and enable 'Remove Duplicates'
Enter the desired value into the 'Duplicate Values' dialog.
Ok out, give your new style a name, then close the 'Style Manager'.
Open the 'Label Manager' from the labeling toolbar.
Select 'Label styles' from the bottom-right of the dialog.
The style you just created should be at the top of the list, double click to apply it.
This should set the properties of the label class to the properties set from the Style Manager.
I hope this helps.