Separate IDocs based on condition

My scenario is XML file to IDoc. In my XML file I have BELNR which will change often.

Whenever a new BELNR occurs a separate IDoc needs to be created.

For example:

If I get BELNR like following:

100100

Then only 1 IDoc needs to created...

100100

100101

Then 2 IDocs needs to created in target side.

How do I get this in mapping...