Requirement - at least Gliffy 3.0
Open the Gliffy Diagramme in Gliffy via Confluence
Click File -> Export as Gliffy XML
Save the diagramt (to an gxml file) on a drive
Start a new Gliffy Diagram via Confluence
Choose Import a file as a template
Browse to the gxml file
Click Create Document
And You have a new drawing 
As an alternative, You can reference the Gliffy from another page like this:
{gliffy:name=my diagram|space=ds|page=A great new way to share ideas|align=left|size=L}