Identifying duplicate attributes using ArcGIS Desktop?

Instructions provided demonstrate how to use the Field Calculator to identify duplicate field values. Single occurrences and the first occurrence of multiple values are flagged with 0. Duplicates are flagged with 1.

Create a new field. Set the type as short or long integer and accept the other defaults. Right-click the newly created field and select Field Calculator. Select the Python parser. Ensure that the 'Show Codeblock' option is checked. Paste the following code into the Pre-Logic Script Code box:

uniqueList = []
def isDuplicate(inValue):
  if inValue in uniqueList:
    return 1
  else:
    uniqueList.append(inValue)
    return 0

Type 'isDuplicate(!Field!)' in the lower expression box and replace the word 'Field' with the name of the field that contains the duplicated values. Click OK. All duplicate records are designated with a value of 1 and non-duplicate records are designated with a value of 0 in the new field.

from: http://support.esri.com/cN/knowledgebase/techarticles/detail/38700