Dear listers

I have two files that i want to merge.

File 1 includes: id (string), case_id (string), date_var1, group==1
File 2 includes: id (string), case_id (string), date_var1, group==0

File 2 includes more individuals than in File 1.

In File1 id=case_id
in File 2 id not equal to case_id, the case_id is repeated 4 times

I want to merge the case_id's from File 1 to case_id in File 2.

But when I
Code:
use "/File1"
merge 1:m case_id using "/File2"
the id's in File 1 is overwritten by the id in File 2

how can I joint the two files? So that I get 5 lines with the same case_id, where 1 line is id=case_id, and 4 lines are id not equal to case_id

Thank you
Lars