For the moment, there can be only one process instance with a given business key for a process definition. This applies to both runtime and history.
However, in reality this does happen. Take for example a review/approve of a certain document. Sometimes, you'll want to restart that process if the reviewing failed. The current db constraint disallows that.
As such, we will remove the constraint from the db schema. If people want it, they can still apply it. There is no check being done on uniqueness in the code anyway.