Errors occur when a Provisioning or Update synchronization step runs from the Workflow in an environment where accounts have been moved recently on the target domain outside of Quick Connect (e.g. the accounts have been moved between OUs in the target domain).
The Workflow fails to run with error samaccountname already exists. During a sync, attribute constraint violation occurs.
A less frequent symptom is that duplicate accounts sometimes get created if the Provision or Update Workflow completes.
Quick Connect database still has the accounts mapped from the the source domain to the old location in the target domain (e.g. the old OU location), as the accounts were moved. Therefore it cannot create or update accounts in the new location as there are still records in the previous OU location.
Unmap and re-map from the Mapping rules, or re-create the Mapping rule between the source domain and target domain.