If your source data already is ordered by register_date you can simply use the exists function:
Load %userid, register_data, if(exists(%userid),1,0) as is_repeated
If your source data is not ordered by register_date you need to do that first and at the same time you can create the is_repeated field with the previous function.
see attached qvw.
comm104653.qvw 151.8 K