I just ran into this today. I thought we had a "copy" option in the past but could not find it anywhere. Would greatly appreciate this for several of our workflows, the most pressing one is submitting new requests and referencing existing artwork.
It's been a long time this idea is handing in the air, but I think this is a great feature that should be implemented in WF.
Many of our users have to manually download and upload documents from one project into another. This is not the most convenient way of working, when it can be simplified with one "Copy To" button.
Fingers crossed this is soon added into a future product release!!