When running the Table To Domain tool in ArcGIS Pro, the following error is returned:
Error: 160039: The value being added to the CodedValueDomain already exists.
This occurs when there are duplicate values present in the selected field. Domain values must be unique. When duplicate values are present in a field, domain values created by the tool are also duplicated.
Delete the duplicate values
Use the Delete Identical tool to delete duplicate value in a field before running the Table To Domain tool. Refer to ArcGIS Pro: Delete Identical (Data Management) for instructions and more information.
Create a new field with sequential values
Create a new field with incremental values before running the Table To Domain tool to create a field with no duplicate values. Refer to How To: Sort and create a sequentially ordered ID field in an attribute table in ArcGIS Pro for instructions and more information.
The image below shows the tool has run successfully.