To clone a database using Rman we used to require an existing Rman backup, on 11g we can clone databases using the "from active" database option.
In this case we do not require an existing backup, the active datafiles will be used as the source for the clone.
In order to clone with the source database open it must be on archivelog mode. Otherwise we can make the clone mounting the source database, as shown in this example.
These are the steps required to complete the clone:
A step by step example is provided on this file: