Problem: A duplicate feature is returned by the dissolve tool of the GeoProcessing wizard
After dissolving layers based on the values of a field, the output sometimes contains duplicate features.
This is due to the following:
The layer being dissolved includes a record that has no geometry; that is, it has null shape. This could be a result of adding a new record to the layer's attribute table without creating a corresponding shape (point, line, or polygon).
The record without a shape has a unique value for the attribute being used to dissolve the layer. The dissolve operation creates a new feature in the output based on this unique value. Because this feature is derived from a record with no shape, there should be no shape in the output; instead, the dissolve operation populates this category with duplicate features.
Solution or Workaround
Delete from the output duplicate features created by dissolving a record with no shape. The duplicate features are the ones associated with the value in the dissolved output derived from the record with no shape of the input layer.
Identify the records with no shape in the original layer.
Zoom out to the full extent of the map. Use the Full Extent button of the Tools Toolbar.
Make sure all your features are visible.
Click the Select Features tool of the Tools Toolbar
Click and drag a box around all the features
Open the layer's attribute table
Click Options and select Switch Selection. The highlighted records should be all those records that have null or empty geometry.
Identify the value of the field used to dissolve the layer. The value for this record with no shape should be unique.
Click the dissolve output layer and open the attribute table.
Select the record containing the unique value derived from the record with no shape.
Click the Editor Toolbar button of the Main Toolbar.
Click the Editor dropdown menu and select Start Editing.
Press the Delete key on the keyboard to remove the selected record.
Click the Editor dropdown menu and select Stop Editing.