[sv-cc] Question about duplicate callbacks

Hi everyone
I have a question about what behavior you would expect if a user places
the exact same callback request multiple times (with vpi_register_cb()).
That is, on the same reference object, with the same user callback
routine, same user_data pointer, so that the simulator is unable to make
any distinction.

The question came up with respect to value-change callbacks, but I think
it might apply to most if not all of them. For the ACC routine,
acc_vcl_add(), the 1995 LRM says this:

"An object can have any number of VCL monitors associated with it, as
long as each monitor is unique in some way. VCL monitors can be deleted
using the ACC routine acc_vcl_delete()."

Does this imply that attempting to place duplicate VCLs is an error? So
far I haven't found anything on the VPI VCB case. Should it error?
Return the same handle as the first time? Allow multiple identical
completions, so that the same user routine is called the same number of
times as vpi_register_cb was called, with the same user data?

Abi