reuse callID by proxy

Hallo,

I am working on a sip proxy based on JAIN Stack 1.2, can the cloned and
proxyed initial request use the same call-id as the originating one? I
do not think it can and should not, for a proxy is implemented as a
B2BUA(or not?) 

But if other parties on single path (e.g S-CSCF) need to check the
callID for some kind of business logic, a changed callid could break it.
So any idea about it?

urs Lebing