https://community.esri.com/t5/arcgis-pro-questions/calculate-field-with-python/m-p/1040236#M38940 <P>I suggest using a cursor instead:</P><LI-CODE lang="python">from collections import Counter layer = # name of layer or path to feature class with arcpy.da.UpdateCursor(layer, ["NAME", "Field"]) as cur: count = Counter(name for name, field in cur) cur.reset() for name, field in cur: cur.updateRow([name, 1 if count(name) &gt; 1 else 0])</LI-CODE> Wed, 24 Mar 2021 15:53:17 GMT JoshuaBixby 2021-03-24T15:53:17Z https://community.esri.com/t5/arcgis-pro-questions/calculate-field-with-python/m-p/1040107#M38912 <PRE class="ckeditor_codeblock">uniqueList = [] def isDuplicate(inValue): if inValue in uniqueList: return 1 else: uniqueList.append(inValue) return 0</PRE><P><span class="lia-inline-image-display-wrapper lia-image-align-inline" image-alt="qqqq.JPG" style="width: 999px;"><img src="https://community.esri.com/t5/image/serverpage/image-id/9266iDB6367E4209CC4F2/image-size/large?v=v2&amp;px=999" role="button" title="qqqq.JPG" alt="qqqq.JPG" /></span>&nbsp;</P> Wed, 24 Mar 2021 11:22:57 GMT https://community.esri.com/t5/arcgis-pro-questions/calculate-field-with-python/m-p/1040107#M38912 KalSsin 2021-03-24T11:22:57Z https://community.esri.com/t5/arcgis-pro-questions/calculate-field-with-python/m-p/1040236#M38940 <P>I suggest using a cursor instead:</P><LI-CODE lang="python">from collections import Counter layer = # name of layer or path to feature class with arcpy.da.UpdateCursor(layer, ["NAME", "Field"]) as cur: count = Counter(name for name, field in cur) cur.reset() for name, field in cur: cur.updateRow([name, 1 if count(name) &gt; 1 else 0])</LI-CODE> Wed, 24 Mar 2021 15:53:17 GMT https://community.esri.com/t5/arcgis-pro-questions/calculate-field-with-python/m-p/1040236#M38940 JoshuaBixby 2021-03-24T15:53:17Z