I have accomplished this task in the field calculator (thanks to another post here, but I now realize that I need to do it using arcpy).
I have code that perfectly marks the second, third etc. duplicate(s) but I need it to mark all values that have a duplicate. Basically, I can't figure out how to mark the FIRST instance as well as the rest. Here's the code:
import arcpy inFeatures = "thephaseIIIlayer" fieldName = "PhaseIII_ID" desc = arcpy.Describe(inFeatures) fields = desc.fields rows = arcpy.UpdateCursor("thephaseIIIlayer", "", "", "PhaseIII_ID", "PhaseIII_ID A") i = 0 for row in rows: if i == 0: value = row.PhaseIII_ID i += 1 if i > 1 row.SplitMarker = i if row.PhaseIII_ID != value: value = row.PhaseIII_ID i = 1 row.SplitMarker = i -1 rows.updateRow(row) del row del rows