Simply list duplicates values for a field using arcpy

arcpy_duplicates.py

table_rows = []
duplicates = []

layer = "Springs"
fields = ["OBJECTID", "FCode"]

with arcpy.da.SearchCursor(layer, fields) as cursor:
  for row in cursor:
    if row[1] in table_rows:
      duplicates.append(row[0])
    else:
      table_rows.append(row[1])

if duplicates:
  print "\n{} duplicate(s) found (listed by {})\n".format(len(duplicates), fields[0])
  print duplicates
  # Do stuff with duplicates
else:
  print "No duplicates found - all values in {} are unique".format(fields[1])

The code should in the Python window of ArcMap (update values on lines 4, 5).

If needed for use in a standalone script, add:

import arcpy
arcpy.env.workspace = "<path to geodatabase>"