Hello and welcome to the community
Just a quick question - can you perhaps expand on what you mean by "Storage vMotion from an external source"? What is this external source that you are copying to vCenter from?
When using Storage vMotion, if the datastore contains a folder with the same name then a new folder is created suffixed with _x (where x is a number). This means there is no requirement to overwrite the data.
Cannot complete the operation because the file or folder ds:///vmfs/volumes/XXXX.vmdk already exists
Can you also confirm the above message because normally you would go into a folder based on the VMs name, i.e. ds://vmfs/volumes/MyServer/
Kind regards.