ippool and disconnection

[Michele Petrazzo - Unipex]

Hi list,
I have an installation with freeradius 2.1.9, postgresql like db and
pppoe server more than one mikrotik routerboards.
Like now I'm making some test and, a part that all seem to work well
(users can connected to the chain pppoe-server mikrotik -> freeradius ->
db), I see a strange thing inside the radacct table: I have a session
that aren't "closed" (acctstoptime field are NULL) and I have another
session, with the _same_ framedipaddress, opened after the previous one.
The nas value are different.
Looking into both nas, the first that as into the db the session opened,
haven't the pppoe session active and radwho say me that there is no
session for that framedipaddress.
I thought that "someone", before open a session with a specific
framedipaddress see if it's already assigned, but it's not the truth.
I think that I'm in wrong in somewhere, but I don't know.

The question: who forgot to close the session? Or better, do I have to
say to freeradius to close it after a "timeout" or something other?

Thanks,
Michele