A Workflow Id is meant to be a business-process identifier such as customer identifier or order identifier.
A Workflow Id Reuse Policy can be used to manage whether a Workflow Id can be re-used. The Temporal Platform guarantees uniqueness of the Workflow Id within a Namespace based on the Workflow Id Reuse Policy.
It is never possible for a new Workflow Execution to spawn with the same Workflow Id as another Open Workflow Execution, regardless of the Workflow Id Reuse Policy. An attempt to spawn a Workflow Execution with a Workflow Id that is the same as the Id of a currently Open Workflow Execution results in a "Workflow execution already started" error.