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