An error message, “Domain already exists" is returned in ArcGIS Collector (iOS), on updating a domain assigned to a field in an ArcGIS Server feature service.
This issue is not reproducible in the current release of ArcGIS Field Maps. For more information on the potential causes and solutions to this error, refer to https://support.esri.com/en/technical-article/000020922.
解决办法
Modify the coded value domain in the feature service, and close it.
Restart the ArcGIS Collector application, and open the web map.