Currently sandbox does not accept selective copy IF a space with the same key is existing on it. 

This feature request is to allow a new option where if a selective copy is done from Prod to Sandbox, we get the option to merge the data, ie combine Prod and Sandbox data in the destination sandbox. 

Additional Context: 

The scenario here is that users in the sandbox needs to test based on latest data in production. However, they do not want to loose existing data, so on a per project basis, data like work items, project configuration etc should be movable so that admins can have full control over what data is copied over and what is merged into an existing space on the destination sandbox. 

Data centre customers do this via copying and merging DB level data which is not possible in Cloud.