ArcPy Mark ALL duplicated values

Question

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

Answer 1

Here it is, the simplest thing ever and as an added bonus it updates an external table with the information and a replacement value. My only worry is that it only works if there is only one duplicated value and returns the last one. Since my users will likely only be splitting polygons one at a time, it shouldn't be a big deal but it is a limitation for sure.

import arcpy
from arcpy import env

dataset = "THENAMEOFTHEDATASET"
outtable = "C:\SCRATCH\scratch.gdb\MEfreq2"
histtable = "C:\SCRATCH\scratch.gdb\History"
expression = arcgis.rand('Integer 0 10000000')
expression2 = arcgis.rand('Integer 0 10')
randomness= int(expression + expression2)

arcpy.Frequency_analysis(dataset, outtable,["PhaseIII_ID"])

rows = arcpy.da.SearchCursor(outtable, ['FREQUENCY', 'PhaseIII_ID'])

hamsterpants = 0
for row in rows:
    if row[0] >1:
        #print row[1]
        hamsterpants = row[1]
print hamsterpants


cursor = arcpy.da.InsertCursor (histtable, ("PhaseIII_Parent", "PhaseIII_ID"))
cursor.insertRow((hamsterpants, randomness))

del row
del cursor