There may be numerous ways to get unique file name in case of multi-mapping for 1..n scenario.
The way I did it was using %TIME parameter in Filename .
The %TIME parameter specifies time in milliseconds.
Here the last 13 characters i.e. 1507752503785 is time specified in milliseconds which when converted to date becomes
Although SAP help has specified it to be used in Archiving section, but all these parameters do work for the main filename too..
Hope it will save you some time… 😉